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Keywords:

  • transplantation;
  • human leucocyte antigen matching;
  • viral infection;
  • chimerism;
  • PET

Summary

  1. Top of page
  2. Summary
  3. Donor sources
  4. HLA typing: defining mismatch and impact on outcomes
  5. Infectious complications
  6. Chimerism
  7. MRD
  8. 18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging
  9. Perspective
  10. Acknowledgements
  11. References

A number of advances in clinical practice that are considered routine in modern allogeneic transplant programmes lack definitive supporting evidence, partly because they may offer modest incremental benefits that are difficult to demonstrate in a statistically robust manner given the relatively small cohorts of patients who undergo such procedures. Nevertheless, these marginal gains probably contribute therapeutically meaningful overall benefit, particularly when aggregated. We review the evidence for a number of these practices in terms of impact on transplant outcomes, with particular reference to the setting of T cell depletion as widely practiced in the United Kingdom, including high resolution tissue typing, surveillance for and therapy of infectious complications, chimerism-directed immune modulation and more sensitive monitoring for residual or progressive disease.

The concept of incremental improvement driven by the aggregation of marginal gains came into sharp focus once again during the summer Olympics of 2012. Whilst originally outlined by team GB cycling director Dave Brailsford in the context of the philosophy they had taken to translate the additive impact of multiple small improvements into a significant overall gain in performance, the basic ideas could, he posited, be translated across many aspects of life. These concepts hold true in medical practice. It is perhaps increasingly rare for a single change to result in dramatically improved outcomes in an established intervention such as haematopoietic stem cell transplantation, but by focusing on the aspects of care that are somewhat less amenable to the demonstration of statistically ‘significant’ improvements in outcome by themselves, sometimes by virtue of the relatively small incremental improvements delivered by any one strategy in isolation, it may be possible to deliver significant overall gains. There are many examples of improvements in patient outcomes over successive time periods that occurred despite no alteration in the primary intervention. For example, the European Group for Blood and Marrow Transplantation (EBMT) registry data demonstrated dramatic improvements in overall (OS) and progression-free survival (PFS) of patients with myeloma undergoing myeloablative transplants between 1994 and 1998 compared to those performed between 1983 and 1993 (Gahrton & Bjorkstrand, 2000). Such improvements are ascribed largely to advances in supportive care coupled to better patient selection.

Whilst more recent advances in transplantation, such as more widespread use of cord blood donors, have focused more on expanding the potential donor pool to enable transplantation in those who would have previously had no suitable donor, the last major shift in transplantation practice that impacted on transplant outcomes for a large portion of the transplant population was the introduction of reduced intensity strategies in the late 1990s. The initial aim of such approaches was to reduce non-relapse-related mortality, simultaneously improving survival outcomes and facilitating more widespread application in patients with greater co-morbidities, with the additional aim of reducing graft-versus-host disease consequent upon less marked collateral tissue damage and reduction in associated inflammatory cytokine release. There have been a number of excellent reviews of this subject (Deeg et al, 2006; Deeg & Sandmaier, 2010), which most would see as providing more than a marginal gain, and further elaboration is beyond the scope of the current review, which seeks rather to focus on areas of clinical practice in which advances have potentially contributed to outcome regardless of conditioning intensity or donor source per se. Likewise, we have not focused on the possible impact of quality management systems, such as the Joint Accreditation Committee International Society for Cellular Therapy and the EBMT (JACIE; Gratwohl et al, 2011). We have instead focused on a number of factors that might be considered marginal gains and which have contributed to improved outcomes over the past 15 years, such as high resolution tissue typing, surveillance for and therapy of infectious complications, chimerism-directed immune modulation and more sensitive monitoring for residual or progressive disease. These aspects of modern transplant practice, which in many cases lack a strongly supportive evidence base, nevertheless merit consideration as integral components of modern transplant practices.

Donor sources

  1. Top of page
  2. Summary
  3. Donor sources
  4. HLA typing: defining mismatch and impact on outcomes
  5. Infectious complications
  6. Chimerism
  7. MRD
  8. 18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging
  9. Perspective
  10. Acknowledgements
  11. References

The expansion in potential donor sources does merit brief comment. The exploration of alternative donor sources, including mismatched related and unrelated donors and umbilical cord blood (UCB), has undoubtedly increased access to allogeneic transplantation for those patients who might previously have been considered unsuitable due to a lack of an ‘appropriate’ donor. Application of double-unit UCB strategies, combined use of a single-unit UCB and a haploidentical graft, and more recent attempts to purge alloreactive T cells in vivo with cyclophosphamide, have all helped to expand the availability of stem cell transplants (Luznik et al, 2008; Ballen & Barker, 2013), although none but the most ardent supporter would argue that they offer a marginal gain when compared to standard matched sibling allografts. Perhaps more contentious are the debates regarding the relative merits of such alternative graft sources, including debate of the relative merits versus mismatched unrelated donor grafts [see also the section on human leucocyte antigen (HLA)-typing]. Comparisons of outcomes in patients following double-unit UCB transplantation after myeloablative conditioning with those of patients receiving matched related donor and matched or mismatched unrelated donor grafts suggest similar outcomes are achievable in adult patients, with the conclusion that double-unit UCB should be considered when there is no matched donor (Brunstein et al, 2010). In the non-myeloablative setting, comparison of two parallel Clinical Trials Network (CTN) trials of double-unit UCB transplantation and haploidentical stem cell transplantation demonstrated comparable disease-free survival (DFS) at 1 year (Brunstein et al, 2011). Non-relapse related mortality was higher after UCB transplantation, but relapse rates were lower. Longer follow-up and a randomized study are required to further assess the merits of these approaches.

HLA typing: defining mismatch and impact on outcomes

  1. Top of page
  2. Summary
  3. Donor sources
  4. HLA typing: defining mismatch and impact on outcomes
  5. Infectious complications
  6. Chimerism
  7. MRD
  8. 18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging
  9. Perspective
  10. Acknowledgements
  11. References

Whilst improvements in outcomes for unrelated donor transplants have been driven by multiple factors, progress in HLA-typing methodologies and a greater understanding of the impact of allelic, as opposed to antigenic, mismatch on outcomes has undoubtedly been an important contributory factor. As a brief background, HLA antigens are identified as being recognized by alloantibodies induced by pregnancy or transfusions, i.e., serologically. Any given HLA antigen may, however, be encoded by a number of HLA alleles with similar nucleotide sequences, and these allelic differences can be distinguished only by DNA-based typing methods. Transplant donor and recipient pairs with different HLA antigens (‘antigen mismatched’) will always have different alleles, whilst pairs with the same allele will always have the same antigen (‘matched’). Some donors and recipients with the same HLA antigen will, however, have different alleles (‘allele mismatched’). As a general rule, HLA allele mismatches are characterized by amino acid substitutions in the regions of the HLA molecule that bind peptides for presentation to T cells, whereas HLA antigen mismatches are characterized by amino acid substitutions relevant to both peptide binding and contact with T cells. The importance of HLA-matching on transplant outcomes is a reflection both of the capacity of the patient's immune system to recognize the graft (host-versus-graft effect, mediating graft rejection), and of the graft to recognize the patient [graft-versus-host effect, mediating graft-versus-host disease (GvHD)]. Whilst the relative importance of antigenic versus allelic mismatch at individual HLA loci remains the subject of some debate, both types of mismatching at a single antigen/allele have been associated with worse outcomes (GvHD, non-relapse-related mortality, DFS and OS) in the T-replete setting, and most mismatches at individual loci appear equally risky when compared at the allelic versus antigenic level [in the US National Marrow Donor Program database analyses, single HLA-DQB1 and HLA-DPB1 mismatches were well tolerated and subsequent analysis based on matching of the other 8 loci (HLA-A, B, C and DRB1) showed HLA-C antigen mismatches were more risky than HLA-C allele mismatches; Flomenberg et al, 2004; Lee et al, 2007]. There is some variation amongst studies as to the relative importance of differing mismatch combinations on post-transplant outcomes, with more recent attempts to define ‘permissive’ and ‘non-permissive’ mismatches (Petersdorf et al, 2007). Such attempts require validation with alternate data sets, and it becomes clear that there are clinically relevant opposing factors in that a mismatch defined as non-permissive in terms of GvHD risk may associate with lower relapse and appear permissive in terms of PFS or OS. In other words, the permissibility of a given HLA mismatch is partly defined by the locus and partly by the disease risk in terms of survival outcomes. The availability of ‘high resolution’ (allelic) typing therefore allows us to distinguish between donors who may appear equally well matched based on serological typing, potentially improving transplant outcomes. Conversely, however, the greater resolution now means that we are less likely than previously to find an apparently fully ‘matched’ donor for every patient or, stated another way, we can more readily identify the group at higher risk who previously would have had serologically cryptic mismatches, even if we have no other choice in donor. This would be useful if there were an intervention to improve their outcomes. Although not universally practiced in the UK, T-cell depletion is one potential approach to this. There are, of course, many other potential impacts of such strategies (addressed later), which make the relative balance of risk and benefit controversial, but there is little argument that they reduce GvHD risk. Depletion of the graft of T cells in isolation without a balancing increase in host immunosuppression is associated with increased graft rejection rates, and this is frequently addressed in the reduced intensity setting by administering T cell depleting serotherapy [largely alemtuzumab or anti-thymocyte globulin (ATG)] in vivo to the recipient as part of the conditioning therapy. With regard to the impact on HLA mismatch, there was no significant difference in OS between 8/8 matched or one antigen mismatched grafts in a cohort of 144 recipients of T cell-depleted reduced intensity conditioned unrelated donor transplants (Shaw et al, 2005), although a single HLA mismatch was associated with an increased rate of primary graft failure. Similar results were described in a more homogeneously treated cohort of 157 patients in a single institution series comparing 10/10 matched to mismatched transplants (inclusive of the HLA-DQB1 locus), whether matching was assessed at the allelic or at the antigenic level (Mead et al, 2010). A more recent retrospective registry analysis in 727 T cell-depleted unrelated donor transplant recipients showed no significant difference in OS in 7/8 mismatched versus 8/8 matched transplants, and the findings were similar in both recipients of myeloablative (n = 457) and reduced intensity (n = 223) conditioning regimens (Shaw et al, 2010). Serotherapy with ATG may also overcome the adverse impact of HLA mismatch. Mismatch at one or more loci was associated with adverse OS when compared to 10/10 matched cases (HR = 2·43; P = 0·0019) in a series of 114 patients with chronic myeloid leukaemia undergoing myeloablative transplants, but this influence was not significant in those receiving T cell-depleted protocols incorporating ATG (approximately one-third of the cohort; Tiercy et al, 2004).

In summary, high resolution typing may improve outcomes in cases where there is a choice between donors who appear serologically equivalent, and the incorporation of T cell depletion may overcome the adverse impact associated with some levels of HLA mismatch. Whilst the latter strategy may also be associated with favourable GvHD rates, concerns regarding an adverse impact on infection rates and relapse risk persist. These are therefore areas in which further marginal gains may be relevant.

Infectious complications

  1. Top of page
  2. Summary
  3. Donor sources
  4. HLA typing: defining mismatch and impact on outcomes
  5. Infectious complications
  6. Chimerism
  7. MRD
  8. 18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging
  9. Perspective
  10. Acknowledgements
  11. References

Improvements in post-transplant supportive care have probably impacted on outcomes of all allogeneic transplants, though the impact is arguably greatest in those in which immune reconstitution is the most delayed. More refined polymerase chain reaction (PCR)-based screening for viral infection allows earlier detection of ‘reactivation’ or invasive infection. This facilitates more appropriate tailoring of post-transplant immune suppression, or targeting of antiviral therapies. The pathogens that have received the most attention are cytomegalovirus (CMV), Epstein–Barr virus (EBV) and adenovirus (ADV), largely because of a combination of the incidence and clinical significance of infection. CMV and EBV are both herpesviridae that persist as lifelong infections under the control of the host immune system following primary acquisition. Suppression of host immunity, as occurs with allogeneic transplantation, is permissive to uncontrolled viral replication. In the case of CMV, this can manifest with end-organ damage including pneumonitis, colitis, or retinitis. The mortality rates associated with CMV pneumonitis remain high despite modern antiviral pharmacotherapies. In the case of EBV, the clinically significant manifestation is a result of transformation events within the reservoir B cell population, resulting in post-transplantation lymphoproliferative disorders (PTLD), i.e., B cell lymphomas. CMV has a relatively high seroprevalence (infection rates range from 50% to 80% in most populations), and viral DNAemia reflecting uncontrolled proliferation and dissemination occurs in 60–75% of seropositive patients receiving T cell-replete transplants (Hebart et al, 2011; Green et al, 2012) and 70–90% of those receiving T cell-depleted transplants (Chakrabarti et al, 2002), making CMV, at least numerically, the most challenging of these viruses. The negative predictive values of PCR-based assays for the development of CMV-related disease are high, allowing safe targeting of antiviral therapy to those at the greatest risk of disease. Interestingly, there has been a more recent shift away from CMV pneumonitis towards CMV colitis as the major clinical manifestation of CMV disease, with the suggestion that the negative predictive value of PCR assays for gastrointestinal involvement is lower, particularly with late onset disease (Moreira et al, 2010). The positive predictive values are harder to define, as pre-emptive intervention has become routine practice. Historical series indicate CMV pneumonitis occurred in approximately one-third of transplant recipients, suggesting a significant level of over-treatment with pre-emptive strategies based on CMV PCR. Quantitative PCR potentially helps to improve the positive predictive value, but the definition of appropriate treatment thresholds based on viral load remains problematic. Intervention and discontinuation thresholds are largely locally defined, and PCR testing is not yet fully standardized. Furthermore, intervention thresholds are likely to be contextual to the specific transplant scenario. For example, patients with less likelihood of reconstitution of protective T cell immunity, including those with CMV naïve donors or following T cell depletion, may merit earlier intervention than those receiving T cell-replete grafts from CMV seropositive donors in whom stimulation of virus-specific memory populations may confer protective immunity. Likewise, detection later following transplant as a second or third infection episode may not warrant such early intervention, whilst that occurring in the context of severe GvHD and enhanced immune suppression may be considered more sinister. Furthermore, the kinetics of the increase in viral load may be as important as the absolute load, although the coefficient of variation of most PCR assays for viral loads close to the limit of detection may be as high as 30%. Thus, increases of less than 0·5 log10 (or three times the baseline level) may not represent true increases. For these reasons, the definition of a single universally applicable threshold is unlikely to be possible. The suggestion that viral load quantification could be combined with measures of CMV-specific immune reconstitution to further refine strategy is attractive but is not currently routinely achievable, nor validated (Gratama et al, 2010). Despite these considerations, the introduction of sensitive surveillance screening programmes and pre-emptive antiviral treatment strategies is another significant advance. Whilst ‘over treatment’ will still occur to some degree, many patients avoid unnecessary treatment with potentially toxic drugs and CMV disease rates appear similar to those achieved with earlier prophylactic strategies. Though definitive evidence for improved survival outcomes with such strategies is lacking, they can be inferred from the reduction in CMV-related mortality coupled to the less widespread exposure to the antiviral agents that were thought to contribute to the lack of OS advantage in the treatment arm of early randomized studies of antiviral prophylaxis (Boeckh et al, 1996).

Polymerase chain reaction-based monitoring of EBV viral load is less well established than that of CMV. Like CMV, EBV has high seroprevalence (often in excess of 90%). Detectable reactivation rates based on PCR surveillance are generally significantly lower, though highly variable (24–70%) and far in excess of historical incidences of PTLD (0·4–8·1% in the majority of series of T-replete transplants, or T-depleted transplants utilizing alemtuzumab or ATG serotherapy (Hale & Waldmann, 1998; Curtis et al, 1999; Hoegh-Petersen et al, 2011)). Reactivation rates are increased following T cell depletion, and seem lower following alemtuzumab as opposed to ATG, perhaps because alemtuzumab also targets the B cell compartment that harbors the latent EBV reservoir (Hale & Waldmann, 1998; Curtis et al, 1999). Once again, the negative predictive values of PCR-based assays for the development of PTLD seem high, but the positive predictive values remain unclear. The disparity between incidence of detection based on sensitive PCR-based assays and historical incidence figures for PTLD (which are notably significantly lower than for CMV) suggest that the positive predictive value is likely to be lower than that of CMV PCR. When considering predictive values of diagnostic or screening tests, it is important to recognize the influence of the prevalence of disease. Using the same test in a population with higher prevalence increases positive predictive value (i.e. reduces over-treatment). Conversely, increased prevalence results in decreased negative predictive value (i.e. potentially increases under-treatment). Whilst many centres have chosen to intervene based on relatively arbitrary PCR-based viral load thresholds with no clear data regarding the natural evolution of such episodes, similar considerations as those discussed for CMV arise regarding attempts to define appropriate thresholds for intervention. Whilst there is evidence linking viral load to PTLD risk (Holman et al, 2012), there is currently no definitive evidence that pre-emptive intervention, as opposed to targeted intervention, in those with clinical signs of PTLD offers a clear therapeutic or survival benefit. This is in stark contrast to the situation with CMV, where delay of therapy until clinical presentation with CMV interstitial pneumonitis is associated with very poor outcomes. Rituximab has become the favoured first-line therapy for PTLD, and many centres now administer it following detection of EBV DNAemia, even in the absence of any evidence of PTLD. Perhaps unsurprisingly, response rates with such strategies are higher than those for the treatment of established PTLD, as these cohorts includes cases in which DNAemia would have resolved spontaneously without intervention. In one study of alemtuzumab-based conditioning, EBV DNAemia was reported with a cumulative incidence of 40·3% by 2 years post-transplant, with levels above 40 000 copies/ml in 16% (Carpenter et al, 2010). The ‘expected’ incidence of PTLD in this cohort would have been perhaps 2% (Hale & Waldmann, 1998; Peggs et al, 2003a), illustrating that even with an intervention threshold of 40 000 copies/ml, over 85% of patients receiving rituximab were probably over treated. Further refinement based on viral load kinetics or cellular immunity might reduce this, but such strategies are not currently widely practiced, feasible, nor validated (Hoegh-Petersen et al, 2011). Although an element of over treatment has again been introduced, the toxicities appear relatively modest (though not non-existent) and the incidence of fatal PTLD is probably reduced. Given that the incidence of the latter is so low, it is unlikely that a sufficiently powered study will ever be performed to provide definitive evidence of this.

Invasive adenoviral infections can result in hepatitis, pneumonitis and multi-organ failure. The highest incidences have been reported in the context of paediatric and T cell-depleted transplants. Interpretation is, to some degree, confounded by variability in study design (often case-based retrospective studies) and definitions of adenoviral disease. The introduction of PCR-based surveillance strategies potentially allows earlier identification of invasive cases, enabling reduction in immune suppression when possible and introduction of virostatic agents, such as cidofovir, before end-organ damage becomes detectable. In our experience, low levels of viraemia resolve spontaneously in the majority of cases (Sive et al, 2012). Higher levels (in excess of 50 000 copies/ml) responded in the majority to cidofovir in combination with modulation of immunosuppression (ADV-associated mortality 0·9% of the entire population). Similar experience has been reported in paediatric patients following the introduction of PCR-based screening (ADV-associated mortality 1·6% (Kampmann et al, 2005)). Thus, whilst definitive evidence for the impact of PCR-based monitoring for ADV in T cell depleted cohorts is missing, ADV-associated mortality rates reported in the context of such surveillance programmes do appear lower than in historical cohorts.

Attempts to hasten the reconstitution of anti-viral immunity by adoptive transfer of virus-specific T cells have yet to reach mainstream practice and, as such, arguments regarding marginal gains may be considered premature. The majority of experience is with CMV-specific T cells (Walter et al, 1995; Peggs et al, 2003b), largely because this represents the most numerically challenging of the viruses, and the incidence of problematic infections related to both EBV and adenovirus has fallen significantly, as already outlined. Phase II data are encouraging (Peggs et al, 2009), and this is one area where confirmatory phase III data may be forthcoming in the near future, based on the development of selection technologies allowing more rapid and less costly generation of a therapeutic product (Schmitt et al, 2010; Peggs et al, 2011a). The low incidences of clinically significant infections related to EBV and adenovirus in modern transplant practice provide particular hurdles to the development of commercial models for delivery of such therapies unless they can be produced rapidly on an ad hoc basis. It is possible to generate multi-virus specific T cell products (Zandvliet et al, 2010), though financial considerations regarding the size of the problem versus the cost implications of more complex production techniques may ultimately define their clinical utility and widespread availability. Finally, it should be noted that these cellular therapy products may generally be more suitable for managing viral infections in the T-deplete transplantation setting, where viraemia is usually a result of a numerical deficiency in anti-viral T cells that can be rectified by adoptive transfer, rather than being closely correlated with GvHD and the enhanced immune suppression that is required to manage it, as is more commonly the case following T-replete transplantation. In the latter case, the impact of ongoing immune suppression, particularly with lympholytic agents such as corticosteroids, is likely to impact negatively on the expansion and function of transferred cells.

A greater awareness of the potential contribution of fungal infections to mortality has lead to enhanced efforts to target appropriate prophylaxis to patients at earlier time points in their treatment pathway [e.g. during induction chemotherapy for acute myeloid leukaemia (AML); Cornely et al, 2007]. A reduction in pre-existing fungal colonization/infection is probably beneficial both during and after allogeneic transplantation. Newer, less toxic antifungal agents have decreased early mycotic infections and make it feasible to provide long-term fungal prophylaxis in patients at high risk due to chronic GvHD. The reduced prevalence of pre-existing fungal infection, coupled with an effective prophylactic strategy during transplantation, reduces the incidence of proven or probable invasive fungal infection (IFI) to 1–2%, making further comparative evaluation of specific agents in this setting numerically challenging (Marks et al, 2011). This raises another interesting discussion point. Though further marginal gains might be foreseen with some of the newer agents, when does the cost of a marginal gain outweigh the benefit? And how is this evaluated in the absence of definitive evidence from randomized trials?

The application of PCR-based surveillance for fungal infection is less well established than such strategies for viral infections. Aspergillus PCR has not previously been included in the European Organization for Research and Treatment of Cancer (EORTC)/Mycoses Study Group (MSG) definitions because it is not deemed adequately standardized or clinically validated. Other assays, such as the galactomannan assay, have been more extensively evaluated. In both cases the issues relating to incidence, sensitivity and specificity are relevant. Most UK transplant centres use mould-active azoles for prophylaxis, and most have rapid access to high-resolution diagnostic computerized tomography (CT) imaging facilities. In the setting of a low background IFI incidence and a negative CT scan result, the positive predictive value of a serum galactomannan falls, and is perhaps insufficient to trigger an intervention that may be equally effective if delayed. In the group of patients with a CT scan report that is suggestive of possible IFI, the incidence of IFI is higher, the negative predictive value of the galactomannan test lower, and it is questionable how many physicians would rely on it to withhold treatment. Such considerations will, however, vary according to local factors. So, for example, in centres where the incidence of IFI is higher which choose not to use a mould-active prophylactic, management algorithms incorporating galactomannan and/or PCR-based surveillance may offer further marginal gains, at least in terms of managing expenditure on antifungal drugs (Morrissey et al, 2013). Careful consideration of the cost implications of implementation of such surveillance would, however, also need to be factored into pharmaco-economic models.

Chimerism

  1. Top of page
  2. Summary
  3. Donor sources
  4. HLA typing: defining mismatch and impact on outcomes
  5. Infectious complications
  6. Chimerism
  7. MRD
  8. 18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging
  9. Perspective
  10. Acknowledgements
  11. References

Another area where practice is evolving is the use of haematopoietic chimerism to direct post-transplant immune intervention. Methods to establish the chimerism of cells from peripheral blood and/or bone marrow have application both in identifying impending rejection and also potentially early recurrence of disease, and can also be employed to target intervention to minimize subsequent relapse risk. An allogeneic graft-versus-malignancy (GvM) effect is presumed central to the efficacy of the reduced intensity approach, and the presence of non-malignant host haematopoeisis is taken as evidence of immunological tolerance and thus predisposition to relapse (Mackinnon et al, 1994; Childs et al, 1999). The use of serial chimerism assays to detect such recipient haematopoeisis can therefore direct administration of donor lymphocyte infusions (DLI), to augment the graft-versus-host response and promote conversion to full donor chimerism (FDC). The expectation would be that this translates into reduced relapse risk.

The area remains contentious. While the identification of declining levels of donor CD3+ T cell chimerism has been consistently associated with an increased risk of disease relapse, debate continues on the significance of stable mixed chimerism (MC) in T cells, with several groups describing no excess of relapse relative to that seen in the FDC group (van Besien et al, 2009). This is perhaps counter-intuitive, given the acknowledged associations both between full donor T cell chimerism and risk of acute and chronic GvHD, and between significant GvHD and reduced relapse risk. Of note, there is substantial variation in the pattern and timing of changes in chimerism observed post-transplant, influenced by conditioning, presence of T cell depletion, donor source and various recipient characteristics, including disease type, which may contribute to the disparities seen in the published series (van Besien et al, 2009; Thomson et al, 2010), all of which have a degree of heterogeneity.

There seem to be several patterns of chimerism, at least in the T-deplete setting. There may be early FDC in T cells, associating with increased acute and chronic GvHD and, in some cases, increased mortality because of this (Lim et al, 2007), or early MC in T cells, protecting against excess GvHD and either then leading to declining donor T cell chimerism with subsequent rejection or relapse, gradual conversion to FDC without intervention, or a state of stable MC in T cells. The pattern of gradual conversion to FDC has been reported to correlate with minimal early toxicity, while being associated with low recurrence rates, although this may not be reproducible in all disease types. Declining donor T cell chimerism is consistently associated with poor outcome, either graft rejection or disease recurrence, again influenced by underlying disease type. Possible management strategies include rapid withdrawal of immune suppression or early administration of DLI. This has been reported several times in the setting of myeloid malignancy (van Besien et al, 2009), and the results are generally interpreted as being favourable, in that those who received intervention fared better than those who didn't. The latter group, however, usually includes significant numbers of patients who progressed too rapidly for intervention and are therefore clearly of higher risk. That said, there are indications that intervention can reverse declining T cell chimerism and that the relapse risk thereafter mirrors that of those already full donor, which can be interpreted as supporting a GvM effect. The issue of strategies predicated on the assumption that immune intervention will be beneficial is a significant problem in this area, as genuine internal control groups are lacking, and comparison to historical data is flawed. A protocol that mandates an intervention which can take some months to induce an effect, may at least partially select for those patients who are destined not to relapse, particularly given that the median time to relapse for most diseases is within 6–9 months. An important study details the alternate approach, where T cell chimerism was monitored following reduced intensity transplantation, but no intervention performed in the face of T cell MC (van Besien et al, 2009). They found declining chimerism to predict for relapse, while there seemed no excess of relapse in those with stable MC.

We have pursued an aggressive strategy of DLI in patients with mixed T cell chimerism following transplantation with an alemtuzumab-containing reduced intensity regimen. Disease-specific analyses of the impact of such intervention have been performed in both follicular and Hodgkin lymphoma and suggest a benefit in terms of reduction in relapse risk without an increase in GvHD sufficient to counterbalance (Thomson et al, 2010; Peggs et al, 2011b). Possible confounding factors include the relatively small numbers of patients analysed, with patient characteristics potentially skewing results, and the possibility that if converting stable MC has no effect on relapse risk, we may still see an apparent affect as those not destined to relapse remain eligible for escalating DLI and may convert, skewing the FDC group. The relapse events in patients with follicular lymphoma who did not convert to FDC occurred beyond the point that conversion was occurring in others, suggesting that this is not the case.

For myeloid malignancies, approaches differ and the evidence is conflicting. In the absence of frank relapse, DLI have similarly been used to convert MC in the T cell lineage with the aim of breaking tolerance and reducing relapse risk. In addition, there is interest in utilizing lineage-specific chimerism for the detection of minimal residual disease (MRD). This reflects the poor long-term outcome observed following immune intervention for frank relapse of AML/myelodysplastic symdrome (MDS) post-allograft. Analysis of chimerism in CD34-expressing cells has provided a platform for preemptive immunotherapy (Rettinger et al, 2011; Rosenow et al, 2013). The relapse rate in patients with AML/and stable donor cell chimerism appears significantly lower than in those with mixed chimerism in unsorted bone marrow or CD34+ cells, resulting in improved survival. Immune modulation in those with mixed chimerism (either withdrawal of immunosuppression or administration of DLI) resulted in conversion to FDC in many, and the bulk of the relapses occurred in those who failed to convert. This suggests successful salvage of impending relapse by immune intervention, but there is no formal indication of relapse timing, so the skewing from those relapsing early, precluding intervention, is an unknown.

There is less evidence that MC in the T cell lineage has significance in terms of relapse risk in patients with AML/MDS (Lim et al, 2007; Nikolousis et al, 2013). Observational studies reporting cohorts that did not receive an intervention based on chimerism analyses are of particular interest, as the lack of intervention removes the potential bias that may be inherent in all other series with an active immunotherapy strategy. In one such cohort of 120 patients with predominantly myeloid malignancy who underwent reduced intensity transplantation using alemtuzumab-containing regimens, MC was associated with a significant reduction in chronic GvHD but no significant overall increase in relapse (van Besien et al, 2009). A subgroup was defined with declining chimerism, in whom relapse risk appeared elevated. Limitations may include the small numbers at risk, in a cohort with substantial heterogeneity in patient characteristics. In addition, the declining chimerism group represented a relatively high proportion of the assessable patients (61% by day 365). Such a strategy of universal non-intervention presupposes that immunotherapy in the presence of MC would not salvage some of those whose chimerism is going to decline and in whom relapse risk is elevated. In fact, the authors postulated that targeted intervention for those with declining chimerism is indicated. It then becomes a balance of risk between intervening unnecessarily in a cohort destined to have stable MC and potentially no relapse, and intervening early enough in those whose chimerism is going to decline and who are therefore in the high-risk category. In the absence of a mechanism for predicting the future course for individual patients, a strategy of intervention could be argued for all, particularly as median relapse is often 6–9 months post-transplant and the group with stable MC was relatively small.

A final comment on the observation that relapse in stable MC seems similar to that in FDC in a number of series relates to the sensitivity of the chimerism assays. Stable MC tends to occur more frequently with relatively high levels of donor chimerism and relatively low levels of recipient chimerism. FDC is variably defined in different series, but usually as >90–95% donor CD3+ T cells. The presence of a significant number of patients with low level mixed chimerism i.e. <5–10% (which has the same significance in terms of bi-directional tolerance as higher levels if stable) within the cohort labeled FDC, may dilute genuine differences in risk, particularly given the small patient numbers involved in most series.

In summary, although this area remains contentious there is some evidence that in certain settings, intervention based upon chimerism analyses can reduce relapse incidence, potentially providing a further clinically significant gain in a subset of patients.

MRD

  1. Top of page
  2. Summary
  3. Donor sources
  4. HLA typing: defining mismatch and impact on outcomes
  5. Infectious complications
  6. Chimerism
  7. MRD
  8. 18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging
  9. Perspective
  10. Acknowledgements
  11. References

As outlined in the preceding section, lineage specific chimerism can potentially be used as a direct marker for MRD, particularly if applied to selected subsets of cells, such as the CD34-expressing compartment of the bone marrow. Other approaches to monitoring MRD, such as PCR for disease-specific translocations, clone-specific T cell receptor or immunoglobulin gene rearrangements, or flow cytometry for combinations of disease-specific cell surface phenotypic markers are also well established. For the majority of these, the evidence that MRD predicts for worse outcome than the absence of MRD seems clear (Perez-Simon et al, 2002; Richardson et al, 2013). Furthermore, as with chimerism-based analyses of CD34-expressing cells, quantification generally confirms that rising levels of MRD correlate with impending relapse. The evidence that intervention based on MRD monitoring improves outcomes is less robust, and most reported series suffer from small numbers of patients and the same sort of interpretation bias that can be introduced by statistical modelling of a variable that changes with time. Such analyses can be complicated and are confounded by the poor outcomes in the groups with MRD who relapse too quickly for intervention. These considerations apply when the outcomes of those converting to MRD negativity are compared directly to those failing to convert to MRD negativity. Beyond this, it remains contentious in many cases whether intervention at a time of MRD necessarily improves outcomes as compared to intervening at a later time point in any given individual. Perhaps the most convincing data supporting early intervention comes from chronic-phase chronic myeloid leukaemia (CP-CML), where lower doses of lymphocytes could affect the same response with a lower likelihood of side effects, such as GVHD or aplasia (Van Rhee et al, 1994), but the introduction of tyrosine kinase inhibitors has greatly reduced the number of allogeneic transplants performed for chronic CP-CML. This is certainly one area where well-designed prospective collaborative trials could help to define future practice.

18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging

  1. Top of page
  2. Summary
  3. Donor sources
  4. HLA typing: defining mismatch and impact on outcomes
  5. Infectious complications
  6. Chimerism
  7. MRD
  8. 18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging
  9. Perspective
  10. Acknowledgements
  11. References

Another area of interest is the use of functional imaging, which can also potentially be used to target immune modulation earlier in relapse. FDG PET is well-established in staging at diagnosis and in monitoring response to primary chemotherapy in various types of lymphoma, and has also been shown to have predictive value when performed pre-autograft in Hodgkin and non-Hodgkin lymphomas [reviewed in (Johnston et al, 2008; Poulou et al, 2010)]. There are several areas where its potential impact in allogeneic transplant has been examined: to predict outcome of allograft according to pre-transplant status and therefore refine patient selection; to improve salvage rates by scheduled use post-transplant for early detection of disease recurrence/progression and to avoid unnecessary intervention in those with residual metabolically inactive CT abnormalities; and, most recently, to stratify risk post-salvage chemotherapy and therefore direct choice of high dose procedure.

There are conflicting data as to whether pre-allograft PET status predicts for outcome. A prospective trial of combined modality PET-CT and CT scanning in 80 patients with lymphoma undergoing alemtuzumab-containing reduced intensity allografts at our institution demonstrated no difference in outcome between the 42 patients with residual FDG-avid lesions pre-transplant and the 38 in a metabolic complete response (Lambert et al, 2010). There is substantial data demonstrating that outcome following reduced intensity allograft in chemo-refractory patients is worse than those who attain at least a partial response (PR) by CT criteria, but this study focused on those who were chemo-sensitive by CT criteria, differentiating those who were or were not in metabolic remission. Relapse risk, non-relapse-related mortality and OS were all statistically equivalent, and there were no significant differences in patient characteristics between the two groups. However, a retrospective study reported a significant association between PET-positivity pre-transplant and relapse in 80 patients with chemo-sensitive lymphoma (Dodero et al, 2010). No such differentiation was apparent when comparing complete response (CR) versus PR groups by CT criteria. This study was retrospective, and relapse was also influenced by donor type in multivariate analysis, but the groups were otherwise well matched. No definitive comments can therefore be made about the predictive value of PET pre-reduced intensity allograft to further refine the chemo-responsive population at this time, but it could be postulated that those who have attained at least a PR by CT criteria may have equivalent outcomes long-term, irrespective of whether the response is complete. The demonstration of chemo-sensitivity should translate into a higher chance of further cytoreduction with conditioning chemotherapy, and there is no a priori reason to expect an allogeneic GvM effect to be predicted by the difference between metabolic CR or PR pre-transplant.

Other studies have evaluated the use of functional imaging as a restaging or surveillance strategy post-allograft, to detect recurrence or progression as early as possible and therefore optimize the chance of immune intervention being successful without the need for substantial debulking, and to evaluate metabolic activity of residual abnormalities detected by CT. In the prospective study by Lambert et al, recurrence was identified with greater sensitivity on PET-CT scan, 39% of patients with relapse by PET-CT criteria having negative concurrent CT reports (Lambert et al, 2010). Following immune intervention, PET-CT was also effective in monitoring response. The study does not by itself indicate that outcomes are improved by earlier intervention. This would require a relatively large randomized study of post-transplant surveillance by PET-CT versus CT, as randomization to non-intervention in the face of a positive PET-CT scan would be problematic. Powering would then have to account for the fact that 60% of cases would receive intervention at equivalent time points, as PET-CT and CT abnormalities occurred simultaneously in this proportion of patients. PET imaging might additionally or alternatively allow avoidance of inappropriate therapy. Of the 19 patients with abnormal CT scans suggestive of possible residual disease that were non-avid by PET (Lambert et al, 2010), only 4 had subsequent relapse within the area of CT abnormality, and 13 remain in ongoing CR. These findings mirror those of an earlier retrospective study of 55 patients with lymphoma from our institution (Hart et al, 2005) and suggest a clinically significant role for PET scanning in assessing for early recurrence or progression, permitting earlier immune intervention in up to 40% compared to CT and avoidance of unnecessary intervention in the majority with residual CT abnormalities.

Finally, there is interest in utilizing PET to risk-stratify before high dose consolidation, in an attempt to identify patients predicted to have poor outcomes with autologous transplant, and to target ‘escalation’ to allogeneic transplantation. At our institution, focus has been on response assessment post-salvage chemotherapy in Hodgkin lymphoma to direct choice of transplant modality for consolidation. The outcome of this strategy has recently been reported in a series of consecutive patients with primary refractory or relapsed disease (Thomson et al, 2013). The three-year OS for the allograft group (n = 25) was 88% and current PFS 80%, with corresponding figures of 93% and 86%, respectivel,y in those receiving an autograft having achieved metabolic CR pre-transplant (n = 28). These encouraging data suggest that a PET-based response assessment in patients with Hodgkin lymphoma in whom primary chemotherapy has not been curative can assist in stratifying risk and directing subsequent therapy. This strategy is currently the focus of a prospective UK single-arm phase II study.

Perspective

  1. Top of page
  2. Summary
  3. Donor sources
  4. HLA typing: defining mismatch and impact on outcomes
  5. Infectious complications
  6. Chimerism
  7. MRD
  8. 18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging
  9. Perspective
  10. Acknowledgements
  11. References

We recognize that many aspects of this review represent a personal perspective on current transplant practices both within the UK and beyond. The evidence base for many components of modern transplant practice remains sparse when compared to that available for primary therapy of specific diseases. The reasons for this are multiple. Whilst not to excuse attempts to build on this evidence in prospective trials, the current review attempts to highlight some of the areas where modest gains have probably contributed to improved outcomes. Marginal gains are, by their very nature, not well suited to robust confirmation in randomized studies, particularly when transplant patient numbers are limiting and current clinical practice already so diverse. Inclement economic conditions and new commissioning strategies are unlikely to facilitate the process. An alternative viewpoint might question the financial costs of marginal gains, and when they meet the law of diminishing returns – that is, further addition of more resource to a process results in an initial increasing return that, as more resource is added, will start to tail off. To take the analogy back to the sporting arena, this has been likened to bungee running, wherein a person is tied to a bungee cord and tries to run to snatch a prize at some distance. The initial few metres are easy with little resistance to forward momentum but as the bungee runner reaches the furthest extent of the cord, the effort needed to go further increases, until they are flung backwards. How hard you are willing to try probably reflects the value of the prize, and in transplant practice, of course, that value is high. Nevertheless, resource is finite, and it is beholden to us all to maximize value. Clear focus on which aspects of transplant practice are amenable to evaluation in deliverable clinical studies, coupled to a collective desire to recruit to such studies, should be a priority in coming years.

References

  1. Top of page
  2. Summary
  3. Donor sources
  4. HLA typing: defining mismatch and impact on outcomes
  5. Infectious complications
  6. Chimerism
  7. MRD
  8. 18F-Fluoro-deoxyglucose positron emission tomography (FDG PET) imaging
  9. Perspective
  10. Acknowledgements
  11. References
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