Acquired immunodeficiency syndrome-related systemic non-Hodgkin's lymphoma
Mark Bower, Department of Oncology, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, UK. E-mail: firstname.lastname@example.org
Human immunodeficiency virus (HIV) was first recognized as a human pathogen less than 20 years ago but, over that time, it has infected millions of people, resulting in a global pandemic. The consequence of HIV infection is a relentless destruction of the immune system culminating in the diagnosis of acquired immunodeficiency syndrome (AIDS). The World Health Organization estimated that, at the end of the last century, 18·8 million people had died of HIV/AIDS, with a further 34·4 million people living with HIV. During 1999, an estimated 5·4 million people were infected with HIV. Over 90% of the new HIV infections occurred in developing countries, 10% were children under 15 years of age, half were women and > 50% were aged between 15 and 24 years of age (Joint United Nations Programme on HIV/AIDS, 2000). In most of the established market economy countries, the HIV infection rate peaked in the late 1980s and has declined. Furthermore, a significant reduction in the death rate for people with AIDS has been seen with the introduction of potent combination anti-retroviral drugs. As a result, in these countries there are increasing numbers of people living longer with their HIV with varying degrees of immunodeficiency.
The development of effective anti-retroviral therapies commenced with the introduction of nucleoside reverse transcriptase inhibitors: zidovudine (1987), didanosine (1991), zalcytabine (1992) and stavudine (1994). In the last 5 years, there has been a rapid expansion of the therapeutic armamentarium to combat HIV, including more potent combinations (zidovudine and lamivudine) and new classes of drugs, protease inhibitors (saquinavir, indinavir, ritonavir, nelfinavir) and non-nucleoside reverse transcriptase inhibitors (nevirapine, delaviridine, efavirenz). The introduction of the last two classes in the late 1990s led to the use of combination highly active anti-retroviral treatment (HAART). This advance was associated with profound and sustained suppression of HIV viral replication, and a dramatic reduction in opportunistic infections, AIDS-defining illnesses and mortality among HIV-infected people.
It has long been recognized that both primary congenital immunodeficiency and iatrogenic secondary immunosuppression predispose to non-Hodgkin's lymphoma (NHL). Therefore, it was not surprising that NHL occurs more frequently in people with HIV. In 1985, the Centre for Disease Control (CDC) included high-grade B-cell NHL as an AIDS-defining illness following the description of NHL in 90 men from a population at risk for AIDS (Ziegler et al, 1984). In the era that preceded the introduction of HAART, NHL in HIV-positive patients was 60–200 times more common than in the general population (Beral et al, 1991; Bigger et al, 1996). In addition to a reduction in the incidence of opportunistic infections, HAART has been associated with a dramatic reduction in Kaposi's sarcoma and primary cerebral lymphoma, however, the effects on systemic AIDS-related lymphoma (ARL) are less clear.
A Swiss HIV cohort study observed a substantial relative risk reduction between 1992 and 1994, and 1997 and 1998, for Kaposi's sarcoma, as well as other AIDS-defining conditions, but no significant trend was seen for ARL, implying a continuing risk for patients despite the use of potent anti-retroviral therapies (Ledergerber et al, 1999). These findings have been reproduced in other studies from the US. The US AIDS Clinical Trial Group found that the incidence rates of both Kaposi's sarcoma and ARL were decreasing; however, the incidence of Kaposi's sarcoma was decreasing far more profoundly and consistently (Rabkin et al, 1999a). A group from the San Francisco City Clinic found the incidence of lymphoma between 1993 and 1996 to be unchanged (Buchbinder et al, 1999), while the Multicentre AIDS Cohort Study (MACS) actually found a moderate increase in the incidence of ARL (Jacobson et al, 1999). The international collaboration on HIV and cancer has pooled the results from 20 published worldwide studies on ARL. The adjusted incidence rates suggest a decline in the incidence of ARL from 1992 to 1996 and 1997 to 1999, with a relative risk ratio of 0·58. This decline, which has occurred since the widespread introduction of HAART, is most marked for primary cerebral lymphoma but is also significant for systemic ARL (Beral et al, 2000). Although this meta-analysis suggests a moderate decline in incidence, there remains a significant risk among patients on HAART and a higher risk in those who do not have access to HAART because of economic restrictions or because they are unaware of their HIV status.
Predictors of arl
Recent studies have aimed to identify risk factors for the development of ARL in HIV-seropositive people. Analysis of our cohort of 7840 HIV-positive patients, representing 43 745 patient-years of follow-up at Chelsea and Westminster Hospital has identified three factors in multivariate analysis that are significantly associated with the development of NHL; age, nadir CD4 count and no prior HAART. As the CD4 count falls, the development of lymphoma becomes more probable: if a cut-off of 350 cells/mm3 is used, the level at which treatment is advocated in the British HIV association guidelines (Gazzard & Moyle, 1998), a steep rise in incidence is seen from 0·7 cases per 10 000 patient-years above 350 cells/mm3 to 10 cases per 10 000 patient-years below 350 cells/mm3 (P < 0·001) (Matthews et al, 2000). A case–control study from Australia similarly found that the duration and level of immunosuppression, measured as the time since seroconversion and CD4 count 1 year prior to ARL diagnosis, predicted for the development of ARL. This study also found that markers of B-cell stimulation (high serum globulin and HIV p24 antigenaemia) were risk factors for ARL, but that nucleoside reverse transcriptase inhibitor therapy was not significantly correlated with the risk of ARL (Grulich et al, 2000). In addition to immune function and HAART, anti-herpesvirus therapy and genetic factors may determine the risk of ARL. Acyclovir has mild activity against Epstein–Barr virus (EBV) in vivo and one case–control study has shown that administration of high-dose acyclovir (≥ 800 md/d) for ≥ 1 year was associated with a significant reduction in the incidence of ARL (Fong, 2000). Meanwhile, Rabkin et al (1999b) have studied chemokine and chemokine receptor gene variants in this context. Stromal cell-derived factor 1 (SDF-1) is a polymorphic chemokine and 37% of Caucasian and 11% of black Americans carry the 3′A variant. The SDF-1 3′A variant was associated with a doubling of the risk of ARL in heterozygotes and a quadrupling of the risk in homozygotes. In contrast, the presence of the chemokine receptor variant CCR5 Δ32 is protective against the development of ARL (Dean et al, 1999; Rabkin et al, 1999b). People who carry the CCR5 Δ32 deletion do not express CCR5 and have been found to be resistant to infection with HIV despite frequent exposure. CCR5 is the macrophage co-receptor for macrophage tropic non-syncytium-inducing strains of HIV.
Over 90% of ARLs are high-grade B-cell tumours and within the REAL classification, approximately two-thirds are diffuse large cell lymphomas, while one-third are Burkitt's lymphomas (Ziegler et al, 1984; Knowles et al, 1988; Lowenthal et al, 1988; Ioachim et al, 1991; Roithmann et al, 1991a; Raphael et al, 1993; Carbone et al, 1996). In addition, two uncommon lymphoproliferative diseases that are associated with human herpesvirus 8 (HHV8) are seen in this population; primary effusion lymphoma (PEL) or body cavity-based lymphoma and multicentric Castleman's disease. PEL is characterized by effusions in serosal cavities (pleura, pericardium, peritoneum) in the absence of solid nodal masses. Histologically, PEL is a CD30 (Ki-1)-positive anaplastic large cell lymphoma that expresses an indeterminate immunophenotype with clonal immunoglobulin gene rearrangements, but lacking c-myc, bcl-2 and p53 rearrangements (Nador et al, 1996). The clinical management of PEL does not differ from other histological variants of ARL, although patients frequently have advanced immunosuppression at diagnosis.
Castleman's disease is a rare lymphoproliferative disorder characterized by angiofollicular lymphoid hyperplasia. Two histological variants are recognized: a hyaline vascular variant and a less common plasma cell variant. Castleman's disease has germinal centre hyalinization or atrophy surrounded by concentric layers of lymphocytes with prominent vascular hyperplasia, hyalinization of small vessels and interfollicular sheets of plasma cells and immunoblasts. HIV-associated multicentric Castleman's disease (MCD) presents with constitutional symptoms including fever, weight loss and night sweats, and clinical findings include lymphadenopathy, hepatosplenomegaly and rashes. Investigations frequently reveal microcytic anaemia, hypoalbuminaemia and polyclonal hypergammaglobulinaemia. The differential diagnosis in HIV-positive people is wide and, as a consequence, the diagnosis, which is established histopathologically, is frequently delayed. The optimum treatment for Castleman's disease has yet to be established. Early reports of MCD in seropositive patients suggested a median survival of less than 6 months. However, earlier recognition of the diagnosis and treatment with splenectomy followed by single-agent chemotherapy may prolong survival. In the largest reported series from Paris of 20 patients treated with splenectomy and vinblastine, the median survival was 14 months (Oksenhendler et al, 1996). Many of the paraneoplastic manifestations of Castleman's disease are believed to be as a result of excess interleukin 6 (IL-6) production by the tumour, possibly from the viral IL-6 homologue gene of HHV8. Indeed, high levels of IL-6, IL-10 and HHV8 viral load have been found to correlate with exacerbations in HIV-infected people with MCD (Oksenhendler et al, 2000). Humanized anti-IL-6 receptor antibody therapy has been shown to be a useful therapy for HIV-negative MCD (Nishimoto et al, 2000) and studies are underway in HIV-associated MCD.
A number of molecular and virological studies have aimed to elucidate the pathogenesis of ARL and two themes dominate: (i) the role of herpesviruses, and (ii) immunoglobulin gene translocations. It is thought that reactivation of latent EBV infection coupled with immune stimulation by the HIV virus (Schnittman et al, 1986) leads to stimulation and proliferation of B lymphocytes. During B-cell proliferation, errors in immunoglobulin gene rearrangement may lead to translocations and oncogene dysregulation, resulting in the development of ARL. Moreover, even in the absence of EBV infection, HIV induces the production of inflammatory cytokines that cause B-cell stimulation, proliferation and activation (Nakajima et al, 1989; Emilie et al, 1992; Masood et al, 1995), and cell lines derived from ARL have been found to express cytokines including IL-6, IL-10 and tumour necrosis factor β (Pastore et al, 1999). This alternative route to B-cell proliferation may explain why EBV is absent in around 40% of ARL cases.
Role of Epstein–Barr virus
Primary infection of epithelial cells by the Epstein–Barr virus (EBV) is associated with the infection of some resting B lymphocytes via the binding of EBV major envelope glycoprotein gp350 to CD21 (the C3d complement receptor) on B cells. The majority of infected B cells have latent virus, with a small percentage undergoing spontaneous activation to lytic infection. Approximately 90% of the world's adult population is infected with EBV and, in normal adults, from 1–50 cells/million B cells are infected. HIV-seropositive patients have impaired T-cell regulation of EBV and 10–20 times as many circulating EBV-positive B lymphocytes.
Most infected B lymphocytes have a type 3 latency pattern expressing just 10 of the more than 80 genes of EBV. The expressed genes include up to six EBV nuclear antigens (EBNAs) as well as latent membrane proteins (LMPs) 1, 2A and 2B, and two non-translated RNAs (EBERs). The roles of these latent genes include maintenance of the episomal DNA, growth and transformation of B cells, and evasion of the host immune system. After infection, the EBV linear genome becomes circular by the annealing of the long-terminal repeat sequences to form an episome. EBNA-1 binds the viral DNA, thus maintaining the episomal structure in the cell cytoplasm. Growth and transformation of infected B cells is chiefly attributable to LMP-1, which acts as an oncogene. LMP-1 mimics constitutively switched-on CD40, a receptor for tumour necrosis factor (TNF). LMP-1 binds and activates cytoplasmic TNF receptor-associated factors (TRAFs), leading to activation of the transcription factor nuclear factor (kappa) B (NFκB), culminating in cellular proliferation. There is evidence of activation of this cellular signalling pathway in EBV LMP1-expressing ARL (Liebowitz, 1998). A number of viral products coordinate the evasion of host immune defences to preserve latently infected B cells. BARF1 is a viral RNA that is secreted and acts as a soluble colony stimulating factor 1 (CSF-1) receptor acting as a decoy for CSF-1 and thereby reducing interferon-α synthesis in response to CSF-1. Similarly, BRCF1 is a viral homologue of IL-10 and inhibits interferon-γ synthesis. BHRF1 and BALF1 are viral homologues of the anti-apoptotic protein bcl-2, contributing to resistance to cytotoxic T cells, and natural killer (NK) cells, which destroy virus-infected cells by inducing apoptosis. Finally, EBNA-1 interferes with antigen processing for presentation by MHC class I. EBNA-1 includes a Gly-Ala repeat motif that prevents its proteosomal degradation. This is a necessary step prior to peptide translocation by transporters associated with antigen processing (TAP) molecules to the endoplasmic reticulum, where they contribute to the assembly of MHC class I molecules. Through these mechanisms, lifelong latent infection of lymphocytes by EBV is achieved.
Some B cells appear to switch to a more restricted latency 1 pattern, expressing only EBNA-1 (a nuclear antigen), and these cells may evade destruction by cytotoxic T lymphocytes and persist. They are believed to be the origin of EBV reactivation and EBV-positive NHL. Indeed, EBV genomic terminal analysis of episomal DNA has shown clonal EBV infection in ARL, implying that EBV infection precedes clonal expansion. HIV-associated diffuse large cell and immunoblastic lymphomas are frequently associated with Epstein–Barr virus. The EBV genome in these lymphomas expresses latency type 3 antigens including EBNA-2 and LMP-1 and -2. In addition, cellular genes are expressed, including the cellular adhesion molecule ICAM-1 (intercellular adhesion molecule 1 or CD54), the integrin LFA-1 (lymphocyte function-associated antigen 1 or CD18) and the addressin LHR (ligand lymphocyte homing receptor or CD44). However, the EBV genome can be detected in only 60% of HIV-associated large cell lymphomas (Kersten et al, 1998) compared with almost all cases of post-transplantation NHL. This implicates other factors in the aetiology of these malignancies associated with HIV infection, including polyclonal B-cell expansion and impaired T-cell immunosurveillance. In contrast, EBV is present in around 30% of HIV-associated Burkitt's lymphoma cases and, in these tumours, EBV is found in the latency type 1 profile expressing only EBNA-1 and small abundant non-translated RNAs (EBERs) (Kersten et al, 1998). Two genotypes of EBV have been described based on nuclear antigen polymorphisms, types 1 and 2. It has been shown that infection or superinfection with the rarer EBV type 2 does not determine the development of ARL in HIV-seropositive people (van Baarle et al, 1999).
Role of human herpesvirus 8
Human herpesvirus 8 (HHV8) is a gammaherpes virus that was originally identified in 1994 from Kaposi's sarcoma lesions. It is most closely related to herpes saimiri virus, which induces lymphoid malignancies in New World primates, and Epstein–Barr virus. The HHV8 genome consists of a 140 kb unique coding region flanked by terminal repeat sequences. The unique region includes at least 81 open reading frames that potentially encode proteins. In addition to structural proteins and viral enzymes, HHV-8 encodes at least 11 pirated eukaryotic cellular proteins that could contribute to tumour formation. These include homologues for a G-protein-coupled receptor (ORF74), cyclin D (ORF72), interleukin 6 (ORF K2), bcl-2 (ORF16), interferon regulatory factor (ORF K9) and CC chemokines (ORF K4 and ORF K6). A number of these genes have demonstrated transforming properties either in transfected cells or culture systems and the viral IL-6 and cyclin D are latently expressed in PEL cells (Cesarman & Knowles, 1999).
HHV8 has been detected in both PEL and multicentric Castleman's disease in seropositive patients but not other forms of ARL. All PELs are associated with HHV8 infection and tumour cells carry a high HHV8 viral copy number (Cesarman et al, 1995). In addition, many PELs are co-infected with EBV, making the relative roles of the two herpesviruses in the pathogenesis of PEL difficult to determine. Certainly, HHV8 expresses a limited repertoire of latent phase genes in PEL cells and these could play a role in cellular transformation. The association between HHV8 and PEL has helped to identify PEL as a separate disease entity, and also allowed the isolation of a number of cell lines that have been used to propagate the virus and aid the development of serological assays. A recent study has also demonstrated that HHV8 is specifically associated with a variant of multicentric Castleman's disease with λ light chain-restricted HHV8-positive plasmablasts in the mantle zone of B-cell follicles. There is a high frequency of plasmablastic lymphoma associated with multicentric Castleman's disease and these lymphomas are also positive for HHV8 and λ light chain (Dupin et al, 2000).
Role of MYC
Marked polyclonal B-cell proliferation is a feature of HIV-associated immunosuppression but not other forms of immunosuppression, and this may be linked to the fact that Burkitt's lymphoma (BL) is associated with HIV but not other forms of immunosuppression. This B-cell proliferation may increase the likelihood of illegitimate recombination during B-cell ontogeny giving rise to immunoglobulin gene translocations. Immunoglobulin gene translocations occur at the sites involved in normal recombination: the J segments involved in VDJ recombination and also the S regions involved in isotype class switching for the heavy chain gene. The consequences of immunoglobulin gene translocations are dysregulated expression of the incoming gene under the influence of the potent B-cell enhancers. In addition, mutation of the incoming gene is frequent on account of the high somatic mutation rates in B cells at the immunoglobulin gene loci, a mechanism that encourages antibody diversity.
Burkitt's lymphoma is characterized by translocations involving the c-myc gene on chromosome 8q24 and immunoglobulin genes. The c-myc oncogene includes a first non-coding exon followed by two coding exons. Myc encodes a transcription factor that forms heterodimers with Max proteins which bind DNA through basic helix-loop-helix leucine zipper motifs. Binding to Max is essential for Myc transforming activity as Myc homodimers are inactive. The Mad family of proteins also bind Max in competition with Myc, and the balance of Myc–Max and Mad–Max dimers forms a homeostatic equilibrium. Both complexes bind the same consensus hexanucleotide DNA motif and influence transcription; Myc–Max complexes favour cellular proliferation while Mad–Max induce differentiation. Myc expression is repressed in quiescent cells, is upregulated by mitogenic stimuli and maintained during the G1 phase of the cell cycle. Myc overexpression thus leads to cellular proliferation and inhibits differentiation, although, in the absence of growth factors, Myc activation also induces apoptosis.
HIV-associated BL resembles sporadic BL in that only 30% of cases are EBV positive and c-myc/immunoglobulin gene translocations are found in most cases. The translocation breakpoints on chromosome 8 occur most frequently in exon 1 or intron 1 of the c-myc gene in ARL rather than 5′ upstream of the gene. This pattern of translocation breakpoints on c-myc does not alter the peptide sequence as exon 1 is not translated, and resembles the breakpoints observed in sporadic BL rather than endemic BL. The reciprocal breakpoint most often found lies in the Sμ portion of the immunoglobulin heavy chain gene on chromosome 14. This pattern, which is also found in sporadic BL, suggests that the mechanism of translocation is defective recombination during isotype class switching of the constant region of the heavy chain of immunoglobulin, which occurs late in B-lymphocyte ontogeny. There appears to be preferential usage of VH3 and VH4 immunoglobulin heavy chain gene segments in ARL. These VH rearrangements include crippling mutations that, for some reason, do not lead to cell apoptosis (Larocca et al, 2000). Similar crippling mutations are occasionally observed in Hodgkin's disease, in which it is thought that escape from apoptosis is mediated through the NFκB pathway.
Role of BCL6
More recent additions to our understanding of the molecular pathogenesis of AIDS-related NHL include the role of bcl-6. The t(3;14)(q27;q23) immunoglobulin translocation in diffuse large cell lymphomas led to the identification of a novel oncogene bcl-6. BCL6 protein is a zinc finger transcription factor that is expressed only in mature germinal centre B cells. It remains uncertain how BCL6 contributes to transformation. This gene is rearranged in 30–40% of diffuse large cell lymphomas and rearrangements have been found in 20% of ARL cases, while mutations of the 5′ regulatory sequences are found in up to 70% of cases (Gaidano et al, 1997a, b). The expression of BCL6 and absence of CD138/syndecan-1 has been used to separate ARL into two groups that are thought to correspond to germinal centre and post-germinal centre origins (Carbone et al, 1998). In contrast, rearrangements of bcl-1, bcl-2 and bax (Gaidano et al, 2000) have not been demonstrated in ARL.
The tumour suppressor gene p53 has a central role in cell cycle control and, hence, regulates cell replication. About 40% of ARL cases have been found to have mutations of the p53 gene. These mutations are found most commonly associated with the small non-cleaved cell or Burkitt-like variants rather than the diffuse large cell histologies. In contrast, mutations of the retinoblastoma gene, which is also a cell cycle regulator, have not been found and ras gene mutations occurred in only 15% of ARL cases (Ballerini et al, 1993). Although T-cell NHL is not an AIDS-defining diagnosis, the multistate AIDS-Cancer registry linkage has provided evidence of an increased incidence in HIV-positive people, with a relative risk of 15. T lymphocytes are targets for HIV infection and retroviral integration, and there is evidence in the few cases of T-NHL that have been analysed at the molecular level that, in these tumours, the HIV integration site lies adjacent to the c-fes oncogene (Herndier et al, 1992; Shiramizu et al, 1994). The fes (feline sarcoma virus) oncogene encodes a tyrosine kinase that is stimulated by cytokines including interleukin 3 and granulocyte macrophage colony-stimulating factor (GM-CSF), leading to macrophage activation. The long-terminal repeat (LTR) of the integrated HIV genome acts as an enhancer that upregulates c-fes expression. In this way, HIV may be acting as an insertional mutagen promoting the development of neoplasia, a role that has been well established for other retroviruses.
ARL frequently presents with advanced stage and/or extranodal disease and B symptoms, and the differential diagnosis in immunocompromised patients often includes opportunistic infections such as Mycobacterium avium complex, cytomegalovirus infection and cryptococcosis. The most common site of extranodal ARL is the gastrointestinal tract, particularly the small intestine, stomach and perianal region, and Waldeyer's ring. Hepatic involvement occurs in up to one-quarter of patients and results have suggested that the prognosis is particularly poor (Benmiloud et al, 1999). Up to 20% of patients have bone marrow involvement by NHL. In addition, HIV infection itself is associated with trilineage abnormalities of haematopoiesis. This is thought to be chiefly owing to HIV infection of bone marrow stromal cells leading to altered haematopoietic growth factor production and abnormal growth of committed progenitor cells (Scadden et al, 1992; Moses et al, 1996). Moreover, there is some evidence that HIV infects the haematopoietic progenitor cell (colony-forming unit-granulocyte/erythroid/monocyte/megakaryocyte or CFU-GEMM) itself (Folks et al, 1988). The trilineage myelodysplasia adds to the myelotoxicity of cytotoxic chemotherapy for ARL.
In addition to HIV-associated primary cerebral lymphoma, central nervous system involvement by ARL is frequent. Leptomeningeal disease may be present at diagnosis and is asymptomatic in up to 20% of patients (Levine et al, 1991a); hence, all patients should have a diagnostic staging lumbar puncture. The cerebrospinal fluid is also a common site of relapse, and prophylactic intrathecal chemotherapy should be administered to patients considered to be at high risk.
Minor differences have been found between patients with HIV-associated BL and diffuse large cell/immunoblastic ARL. In general, patients with BL are younger and have higher CD4 cell counts (Beral et al, 1991; Roithmann et al, 1991a). Bone marrow infiltration and nodal disease is found more frequently with BL, while the diffuse large cell/immunoblastic tumours more often present with extranodal disease involving the gastrointestinal tract and, of course, include all the primary cerebral lymphomas (MacMahon et al, 1991; Roithmann et al, 1991b). There is no significant difference in outcome between the two histological groups in most studies.
The prognostic factors determining survival duration in early studies of ARL were more related to the severity of immunosuppression than directly to the lymphoma. This may have been because the majority of patients present with advanced stage, B symptoms and/or extranodal disease. These factors were therefore less discriminatory with regard to prognosis and, in that era, there were no effective anti-retroviral therapies. Two series revealed that the CD4 count at diagnosis, a history of a prior AIDS-defining diagnosis, a poor performance status and bone marrow involvement were independent adverse prognostic factors in multivariate analyses (Kaplan et al, 1989; Levine et al, 1991b). Following the introduction of an international prognostic index (IPI) for aggressive lymphoma in the non-HIV-infected population (The international non-Hodgkin's lymphoma prognostic factors project, 1993), this scoring system was evaluated in ARL. Prospective data provided by a recent AIDS Clinical Trials Group (ACTG) study suggested that the prognostic variables in ARL closely resemble those in the IPI (Straus et al, 1998). A retrospective cohort analysis confirmed this finding and showed that the degree of immunodeficiency (measured by CD4 cell count) correlated with increasing IPI score (Rossi et al, 1999).
There is some evidence from single institution cohorts that the median survival of ARL is improving (Matthews et al, 2000), and the median survival when HAART was added to infusional chemotherapy exceeds that observed when didanosine alone was used in a large Eastern Co-operative Oncology Group (ECOG) phase II study (Sparano et al, 2000). The improvement in median survival has not been coupled with an improved response rate to lymphoma therapy and probably represents advances in the management of HIV as many of these patients will succumb to HIV-related illness rather than ARL.
The optimal treatment modality for ARL is combination chemotherapy as patients usually have disseminated disease and extranodal involvement at presentation. However, it was recognized early in the AIDS epidemic that most patients did not tolerate standard lymphoma chemotherapy well owing to the HIV-associated immunosuppression, and for many patients with adverse prognostic factors the life expectancy was very short. This led clinicians to explore dose-reduced schedules and adopt a prognosis-stratified approach to the management of ARL. The challenges of chemotherapy in ARL include maximizing lymphoma-free survival without further compromising immune function and minimizing deaths from opportunistic infections. The unique clinical issues in this context are the management of anti-retroviral therapy and the potential drug interactions with chemotherapy, opportunistic infection prophylaxis in the face of declining CD4 cell counts owing to chemotherapy, and central nervous system chemoprophylaxis.
In most centres, patients with advanced AIDS and limited remaining anti-retroviral options are treated with palliative intent using mild chemotherapy and/or radiotherapy. The palliative chemotherapy schedules include a combination of oral prednisolone and intravenous vincristine, often with intravenous bleomycin. Radiotherapy can be used for localized symptomatic lesions. The life expectancy for these patients is 3–6 months, although there have been no studies that address the role of this palliative chemotherapy approach.
During the 1980s, conventional chemotherapy schedules were used at full dosage for patients with better prognostic factors. However, marked toxicity and an increased incidence of opportunistic infections led to modifications of the standard lymphoma regimens, such as the modified mBACOD (methotrexate, bleomycin, doxorubicin, cyclophosphamide, vincristine, dexamethasone) schedule used by the ACTG (Levine et al, 1991a). The subsequent development of haematopoetic growth factors allowed more myelotoxic schedules to be studied. Full-dose mBACOD with GM-CSF support was compared with low-dose mBACOD in a randomized comparison. There was no difference in either response or survival duration, however, an increased incidence of neutropenic sepsis was recorded in the full-dose arm (Kaplan et al, 1995). A number of cohort studies of ARL have been published using chemotherapy schedules based on treatments used for immunocompetent patients that have reported response rates of 33–63% and median survivals of 5–11 months (Gill et al, 1987; Knowles et al, 1988; Lowenthal et al, 1988; Kaplan et al, 1989, 1995, 1997; Levine et al, 1991a; Remick et al, 1992; Gisselbrecht et al, 1993). These studies are not directly comparable owing to variations in the case mix between series and differences in patient selection. Nonetheless, the response rates and survival data for these trials remain worse than those for HIV-negative patients with high-grade NHL.
Apart from the ACTG study mentioned above, there has only been one other randomized control trial of treatment for ARL. The European Intergroup Study devised a risk-stratified trial in which patients were allocated one of three prognostic groups on the basis of CD4 count < 100/ml, a prior AIDS-defining diagnosis and a performance status score > 1. One hundred and eighty-eight good prognosis patients were randomized between ACVB (doxorubicin, cyclophosphamide, vindesine, bleomycin and prednisolone) and CHOP (cyclophosphamide, doxorubicin, vincristine and prednisolone) regimens. Complete response rates were 65% and 56%, respectively, and there was no difference in overall survival. One hundred and thirty-nine patients with intermediate-risk disease (one adverse factor) were randomized to receive either full-dose CHOP or 50% dose-reduced CHOP. Patients treated with full-dose CHOP had a significantly higher complete response rate (59% vs. 35%), although this did not translate into an improved overall survival. It should be remembered that the majority of patients were enrolled onto this study prior to the widespread use of HAART and that only around one-quarter of the patients were receiving HAART at entry. The poor-risk group are randomized between half-dose CHOP and vincristine and prednisolone, but recruitment to this arm is incomplete (Tirelli, 2000).
Following chemotherapy, up to 40% of the patients with good prognostic features will remain in remission until another AIDS-related illness ensues. Recent studies have reported higher median survivals with no change in response rates for ARL (Bower et al, 2000) and it is thought that the improved survival duration is related to reduced deaths from opportunistic infections in the HAART era among patients who have durable remissions of their ARL.
Infusional chemotherapy for high-grade lymphoma including ARL was pioneered at the Albert Einstein Cancer Center in New York using the combination of cyclophosphamide, doxorubicin and etoposide (CDE) administered as a 96-h continuous infusion for up to six courses at 4 weekly intervals, together with granulocyte colony-stimulating factor (G-CSF) (Sparano et al, 1993). Early reports of a selected group of 25 patients with ARL who were treated with CDE and didanosine produced an impressive median survival of 18·4 months and was widely heralded as a breakthrough in the management of ARL (Sparano et al, 1996). Subsequently, the group used the CDE schedule in combination with protease inhibitor-based HAART therapy with similar results, although there was more mucositis with the protease inhibitor (Sparano et al, 1998). It remains to be seen whether the impressive survival with CDE reflects a schedule that truly produces more durable responses on account of the steroid sparing or infusional schedule, or alternatively is as a result of recent improvements in the overall management of HIV disease. A large multicentre phase II trial of infusional CDE has been conducted by the Eastern Cooperative Oncology Group and the results have been far less impressive, as is so often the case following encouraging initial single centre studies (Sparano et al, 2000).
At the National Cancer Institute, a dose-adjusted schedule of EPOCH (etoposide, prednisolone, vincristine, cyclophosphamide, doxorubicin) has been developed that omits all anti-retroviral therapy for the duration of the chemotherapy. Initial reports have been encouraging with a complete response rate of 79%. However, there was a dramatic fall in CD4 cell count during chemotherapy and, even with restarting the HAART at the end of the chemotherapy, this took 12 months to recover to baseline levels (Little et al, 1999).
In immunocompetent patients, chemotherapy induces both myelosuppression and immunosuppression and the latter is dominated by T-cell depletion, although declines in both B lymphocytes and natural killer cells also occur. The recovery of T-cell populations following chemotherapy is chiefly via thymic-dependent development yielding naïve T cells that have undergone novel T-cell receptor gene rearrangement and await antigen binding. These thymic emigrants therefore expand the immunological repertoire and may be identified by either the presence of ‘T-cell receptor rearrangement excision circles’ (TRECs) (Douek et al, 1998) or by CD45 isoform expression. A second thymus-independent pathway of T-cell regeneration may involve the gut mucosa and lymph nodes (Mackall et al, 1993). Thymus-independent peripheral expansion of T cells is thought to occur by mitotic division of mature T cells and, hence, does not increase the diversity of the T-cell repertoire (Mackall et al, 1996). The prolonged T-cell depletion recorded following EPOCH (Little et al, 1999) was previously demonstrated for patients receiving chemotherapy for ARL (Zanussi et al, 1996). A study of concomitant HAART and chemotherapy in 20 patients with ARL has demonstrated that the CD4 cell count declines by 50% during chemotherapy but recovers rapidly within 1 month of completing chemotherapy. The CD8 and natural killer (CD16 and CD56) cell counts follow a similar profile, while the B-cell (CD19) count recovers more slowly but is restored to prechemotherapy levels after 3 months. There was no change in the HIV mRNA viral load during the chemotherapy (Powles et al, 2000). These findings support the use of concomitant HAART during chemotherapy to maintain immune function as much as possible during chemotherapy when patients are at high risk of infection. Moreover, the ability to estimate the nadir CD4 count during chemotherapy has implications for prophylaxis against Pneumocystis carinii pneumonia (PCP) and Mycobacterium avium complex (MAC) infections, in which the risk is related to the absolute level of CD4 cells. Prophylaxis against PCP is recommended for patients with a CD4 cell count < 200/ml and for MAC cases with a CD4 cell count < 50/ml.
Overall, the concomitant use of anti-retroviral agents with chemotherapy is generally acceptable practice, with the exception of zidovudine, which significantly adds to the myelosuppression of combination chemotherapy, and didanosine, which may worsen the peripheral neuropathy caused by vinca alkaloids. Little is known about the pharmacokinetic interactions of protease inhibitors and non-nucleoside reverse transcriptase inhibitors with chemotherapy, although the inhibition of the cytochrome p450 enzyme system may reduce hepatic metabolism of cyclophosphamide and the anthracyclines.
There is a high rate of meningeal involvement in ARL and it is not necessarily associated with bone marrow involvement or a poor prognosis. Although the frequency of meningeal relapse can be reduced by the use of prophylactic intrathecal chemotherapy, this necessitates repeated lumbar punctures. Intrathecal chemotherapy (methotrexate or cytosine arabinoside) should therefore be given to patients with meningeal disease or at high risk of cranial disease by virtue of Burkitt's histology or extensive paranasal sinus and base of skull disease. Most centres also recommend prophylactic intrathecal chemotherapy for patients with bone marrow involvement and many clinicians feel that this should be standard practice for all patients with ARL. This approach has been questioned by one recent series of 26 patients who were treated with CDE infusional chemotherapy who did not have bone marrow infiltration and received no intrathecal prophylaxis, and none relapsed with meningeal disease (Desai et al, 1999).
Experience with refractory and relapsed ARL has been depressing with occasional false dawns (Levine et al, 1997) but few clinically useful responses. Many clinicians recommend best supportive care in these circumstances. A recent report, however, offers a glimmer of hope. Alexandra Levine's group have reported encouraging results with ESHAP (etoposide, methylprednisolone, high-dose cytarabine, cisplatin) with a response rate of 54% and median survival of 7 months in 13 patients, although the haematological toxicity was considerable (Levine et al, 2000).
The postulated role of herpesviruses in the pathogenesis of AIDS-related NHL has led to therapeutic strategies directed towards these viruses. Latent EBV infection targeting has been developed using EBV-specific cytotoxic T lymphocytes to prevent post-transplant lymphoproliferative disease (Heslop et al, 1999), however, the restricted expression of potential latent antigen targets and the disruption of antigen presentation by EBV (Levitskaya et al, 1995) may limit this approach for ARL. Another strategy directed at latent viral infection aims to disrupt viral episomes by targeting episomal maintenance, as loss of episomal EBV has been found to lead to apoptosis in some Burkitt lymphoma cell lines (Komano et al, 1998; Ruf et al, 1999).
Tumour cells with lytic virus infection express viral thymidine kinase and are therefore rendered sensitive to the prodrug ganciclovir, which requires initial phosphorylation by viral thymidine kinase. Induction of viral lytic cycle in tumours is being explored for both EBV- and HHV8-associated tumours using histone deacetylase inhibitors, such as butyrates for HHV8 and 5-azadeoxycytadine for EBV, which induce lytic phase in vitro (Cannon et al, 2000). Moreover, 5-azadeoxycytadine increases EBNA-2 expression in latently infected cells, which are a target for cytolytic T lymphocytes, and may allow tumour rejection by host immune system.
Monoclonal antibody conjugate therapy was initially investigated for refractory ARL using a ricin-bound mouse-derived IgG1 monoclonal antibody B1 (antiCD19) (Tulpule et al, 1994). CD19 is expressed on normal and malignant B lymphocytes. Promising results were reported when the antibody conjugate was used in combination with chemotherapy, although the contribution of the immunoconjugate cannot be separated from that of the chemotherapy (Scadden et al, 1998). In addition, monoclonal antibodies to IL-6 have been evaluated with limited success (Emilie et al, 1994) and studies of rituximab, a humanized monoclonal antibody to CD20, are underway in this group of patients (Barrett et al, 1999). Finally, anti-idiotype vaccination for B-cell NHL has been proposed as a strategy for at least 20 years and active immunization of patients with tumour idiotype-derived single chain antigen coupled to chemokines to enhance the immunogenicity is in development (Bendandi et al, 1999).
The multidrug resistance (MDR) transporter protein P-glycoprotein (P-gp) has been found to be overexpressed in 70% of newly diagnosed ARL and expression of P-gp was associated with a lower remission rate (Tulpule et al, 2000). Protease inhibitors are substrates for P-gp and overexpression is a recognized mechanism for resistance. In this study, there was, however, no correlation between protease inhibitor usage and P-gp expression by ARL. Inhibition of P-gp with various modulators is an attractive goal for many tumour types in which this mechanism is invoked to account for drug resistance including ARL.
ARL will become an increasingly frequent diagnosis with the rapidly rising incidence of HIV throughout the world. The clinical management requires expertise in both the chemotherapy of high-grade lymphomas and the treatment of HIV, including anti-retroviral therapy and opportunistic infection management. These considerations reinforce the need for patients to be treated in specialized units with the appropriate levels of expertise. The modest improvements in survival that have been achieved recently for ARL are chiefly related to this multidisciplinary approach that combines the talents of HIV physicians and oncologists. Finally, the lessons from the pathogenesis of ARL may also lead to better comprehension of post-transplantation lymphoproliferation, an illness that seems bound to increase with the increasing usage of allograft transplantation.