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Keywords:

  • Dendritic cell;
  • subsets;
  • rat;
  • migration;
  • lymph;
  • tolerance

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. LYMPH DC
  5. PERIPHERAL DC
  6. DC IN LYMPHOID TISSUES
  7. DISCUSSION
  8. CONCLUSIONS
  9. REFERENCES

Dendritic cells (DC) comprise phenotypically-distinct subsets that sub-serve distinct functions in immune induction. Understanding the biology of DC subsets in vivo is crucial for the understanding of immune regulation and its perturbations in disease. This review focuses on the phenotype and functions of rat DC subsets and compares these with subsets identified in other species. Our research has concentrated on DC migrating in lymph. DC migrate constitutively from peripheral tissues to draining nodes, probably to induce/maintain tolerance to self- or harmless foreign antigens. After removal of mesenteric lymph nodes (MLN) in the rat, healing of afferent and efferent lymphatics permits migrating intestinal DC (iLDC) to be collected from the thoracic duct. We have shown that iLDC consist of least two subsets that differ in phenotype, in situ distribution and function. CD4+/SIRPα+ iLDC are highly immunostimulatory, but are excluded from T cell areas of MLN. In contrast, CD4/SIRPα iLDC are less potent stimulators of T cells, but carry material from apoptotic enterocytes to T cell areas of MLN. Similar subsets exist in both lymph nodes and spleen. It has been shown that phenotypically-similar subsets migrate in skin-draining lymph in cattle and sheep. We and others have shown that splenic CD4/SIRPα DC can phagocytose allogeneic cells in vitro, are poor stimulators of CD8+ T cells, and can lyse NK-sensitive target cells. Although some of our data suggest that rat CD4/SIRPα DC may equate to murine CD8+ DC, there is at present insufficient evidence to be confident of this.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. LYMPH DC
  5. PERIPHERAL DC
  6. DC IN LYMPHOID TISSUES
  7. DISCUSSION
  8. CONCLUSIONS
  9. REFERENCES

Dendritic cells (DC) are rare cells in peripheral tissues. They are bone marrow-derived cells and seed peripheral tissues via the blood. DC migrate from these tissues to the draining lymph node (DLN) via afferent lymphatics. Once DC have entered DLN, unlike lymphocytes, they do not leave in efferent lymph in significant numbers, rather they die in the node. DC exit from peripheral tissues is most probably regulated by expression of receptors for chemokines. The upregulation of CCR7 that enables chemotaxis towards constitutively expressed CCL19 and 21 in the lymph node is the best defined mechanism. Migration from the periphery to the DLN is accompanied by a set of cell biological changes in the DC. DC in the periphery are highly phagocytic and pinocytic but downregulate these functions by the time they reach the DLN. In addition, DC redistribute intracellular MHC class II to the cell surface.

DC act as sentinels of the immune system; encounter with pathogens or tissue damage rapidly stimulates increased exit from the tissue into afferent lymph. In addition, such stimuli stimulate a dramatic increase in surface expression of co-stimulatory molecules and a concomitant increase in stability of MHC-II molecules on the cell surface. This empowers DC to exert their central function of naïve T cell activation. Whether this response will be skewed towards a cell-mediated or humoral response may depend on the subpopulation of DC that interacts with the T cell (1).

Importantly, DC exit from peripheral tissue does not occur only after microbial or inflammatory stimuli but is constitutive. It has become increasingly evident that this steady-state migration of non-activated DC and their interaction with naïve T cells plays an important role in the induction and maintenance of peripheral tolerance. It has been suggested that specific subsets of DC perform this function in both rats and mice (2–5).

We have studied DC in rats for several years, focusing on DC migrating in intestinal lymph. In this review we will discuss our data regarding different subsets of DC and relate our findings to DC subpopulations in other species.

LYMPH DC

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. LYMPH DC
  5. PERIPHERAL DC
  6. DC IN LYMPHOID TISSUES
  7. DISCUSSION
  8. CONCLUSIONS
  9. REFERENCES

As DC do not leave the DLN they can be recovered from afferent but not efferent lymph. To study DC that have left the periphery and are migrating in peripheral lymph we have used a model permitting collection of pseudo-afferent lymph (first developed by Graham Mayrhofer). LN draining the tissue of interest (in our case the mesenteric lymph nodes (MLN)) are excised (Fig. 1). The afferent and efferent lymphatics join during healing, forming pseudo-afferent lymphatics. This enables DC that would otherwise be trapped in the LN to enter the blood stream via the thoracic duct. One to two months after surgery the thoracic duct of rats lacking MLN (MLNX) is cannulated and the pseudo-afferent lymph is collected on ice. These DC are derived selectively from the gut, as DC migrating from any other tissue will be trapped in their DLN. The intestinal lymph DC (iL-DC) are then enriched using density separation followed by MACS purification. This purification method collects DC that have just left the periphery and that have experienced minimal manipulation ex vivo (Fig. 1).

image

Figure 1. Procedure for purification of intestinal lymph DC.

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Subpopulations of iL-DC

Phenotypic analysis of the low-density fraction of intestinal pseudo-afferent lymph showed that there are two distinct subsets of veiled DC (6). One subset had a round cell body with multiple small spiky processes, while the other showed an irregular cell body with long blunt pseudopodia. Flow cytometric analysis of the low-density fraction identified these veiled cells as iL-DC with high expression of MHC class II and selective expression of an αε-integrin (the latter recognised by the mAb OX-62 (7)). Further analysis showed that iL-DC could be separated into two distinct populations by the presence or absence of surface CD4 and SIRPα (the latter recognised by the mAb OX-41) (8). Cell sorting using either of these surface markers correlated well with the microscopic studies. The round-bodied iL-DC with small spikes expressed CD4 and SIRPα, and the subset with irregular cell bodies lacked expression of CD4 and SIRPα. When screening for other markers that might be differentially expressed by the two subsets it was noted that the CD4/SIRPα+ iL-DC had higher surface expression of CD90 than the CD4/SIRPα subset (8).

The time that DC spend in the intestinal wall was estimated by the injection of tritiated thymidine or BrdU, which are incorporated into DNA of dividing precursors in the bone marrow. The first labelled DC appeared in lymph at around 24 h and labelling peaked between 3 and 4 d, when a maximum of 30–40% of DC were labelled (6). Thus, a major fraction of DC is migrating rapidly through the intestine into lymph. These experiments do not, however, exclude the possibility of a longer-lived subpopulation of DC in the intestinal wall. The same rapid turnover has been shown for DC in lung, spleen and lymph nodes (9–11). When comparing the output of DC subsets into intestinal lymph we detected similar kinetics for the appearance of BrdU-labelled CD4/SIRPα+ and iL-DC, suggesting that one DC subset was not the precursor of the other (8). In unpublished experiments we have also shown that one iL-DC subset does not develop the phenotypic characteristics of the other in culture.

The two subsets of iL-DC show markedly different survival in culture. CD4/SIRPα DC have a much reduced viability after 24 h in culture compared to the CD4/SIRPα+ (8). Survival can, however, be slightly increased by GM-CSF. Whether this difference in viability reflects in vivo events in the LN is under investigation.

The two subsets of iL-DC also differ in their function as antigen-presenting cells. This was shown by comparing the subsets as stimulators of a MLR, stimulators of pre-sensitized T cells in vitro, and as activators of naïve T cells in vivo. In all the assays the T cell stimulatory/priming capacity of the CD4/SIRPα DC was much less than that of CD4/SIRPα+ DC (8). Whether this relates to reduced viability in vitro or reduced migratory capacity in vivo of the CD4/SIRPα DC is not yet known.

When the two iL-DC subsets were examined by electron microscopy it was observed that more than 80% of the CD4/SIRPα subset contained one or more inclusions of differing sizes and densities, whereas these were not seen in the CD4/SIRPα+ subset of iL-DC (6, 8). In addition, the CD4/SIRPα DC were strongly positive for non-specific esterase (NSE) activity (3). Lack of or weak NSE activity has been used historically to distinguish DC from macrophages. This prompted us to examine the basis of this activity. The intensity of staining in CD4/SIRPα DC was much stronger than we had observed in any macrophage populations, but we had observed that intestinal epithelial cells were intensely reactive for NSE. We took advantage of the fact that NSE represents different enzymes with different isoforms that display characteristic electrophoretic mobility, revealed by running cell lysates on a non-denaturing gel. The isoform profile is characteristic of different tissues. Using this technique we could show that the CD4/SIRPα iL-DC contained NSE isoforms with mobility similar to intestinal epithelial cells.

Further examination of the inclusions in CD4/SIRPα iL-DC showed that about 30% contained apoptotic DNA. Immunohistology showed that DC containing apoptotic material were present in intestinal villi and that these cells were NSE+. DC with strong NSE activity could be detected in large numbers in PP and MLN, but only very rarely in other LN. In addition, NSE+ DC were primarily found in the T cell area of the MLN. Similar uptake of NSE+ material by DC was seen in the lamina propria and MLN from gnotobiotic rats, showing that there was continuous transport of enterocyte-derived material by iL-DC in the absence of any obvious inflammatory or microbial stimuli. On the basis of these findings we proposed the following model for the induction and maintenance of tolerance to peripheral antigens under steady-state conditions (Fig. 2).

image

Figure 2. CD4/SIRPαL-DC in the lamina propria internalise intestinal epithelial cells that have undergone apoptosis. These L-DC then enter the afferent lymphatics and migrate to the T cell areas of the MLN. Upon interaction with naïve T cells these DC might induce tolerogenic or anergic responses to maintain tolerance to gut-derived antigens.

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What happens to the iL-DC when the system is perturbed and the immune system shifts from tolerance to activation? We have started to study this by using i.v. LPS as a model pro-inflammatory stimulus. After giving LPS we could detect a five-fold increase in the output of iL-DC that peaked at 12 h post injection (12). The enhanced release of the iL-DC into lymph could be largely inhibited by a blocking mAb to TNFα. This suggested that the increased release of DC from the intestine might not reflect a direct interaction between DC and LPS, but could instead be a downstream effect due to TNFα released by another cell type responding to endotoxin. Interestingly, the surface phenotype of the iL-DC did not change dramatically nor was T cell stimulatory capacity enhanced in iL-DC recovered after LPS administration in comparison to steady-state iL-DC (Turnbull et al. unpublished observations).

PERIPHERAL DC

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. LYMPH DC
  5. PERIPHERAL DC
  6. DC IN LYMPHOID TISSUES
  7. DISCUSSION
  8. CONCLUSIONS
  9. REFERENCES

Most studies have used histology to investigate DC in peripheral tissues. These have identified DC in lung, liver and intestine (13–15). However, none of these studies have divided DC into subsets. Studies of DC enriched from peripheral tissues are very scarce. One exception is rat respiratory tract DC, which have been studied extensively by Holt and his co-workers, and these studies are reviewed by them in this issue of APMIS.

DC IN LYMPHOID TISSUES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. LYMPH DC
  5. PERIPHERAL DC
  6. DC IN LYMPHOID TISSUES
  7. DISCUSSION
  8. CONCLUSIONS
  9. REFERENCES

DC in rat lymphoid organs have been examined by Josien and co-workers (16–18). They have mainly examined DC in the spleen, but have in addition studied DC from LN and thymus. As for iL-DC, splenic DC were identified by high surface expression of MHC class II and αε-integrin. Splenic DC also comprised two major subsets identified by CD4/SIRPα+ and CD4/SIRPα expression (17). Further studies showed that the CD4/SIRPα+ subset selectively expressed CD5 and CD90 (18). Splenic CD4/SIRPα DC survive poorly in culture unless GM-CSF or IL-3 is added, as do their counterparts in intestinal lymph. Splenic CD4/SIRPα+ or – DC stimulated naïve allogeneic CD4+ T cell proliferation to a similar extent. In contrast, when naïve CD8+ T cells were used as responders, CD4/SIRPα were far less potent than CD4/SIRPα+ in inducing proliferation (18).

Similarly to CD4/SIRPα iL-DC, CD4/SIRPα splenic DC contained cytoplasmic inclusions (18). Whether these inclusions contained apoptotic material was not investigated. In preliminary experiments we have only found NSE activity in CD4/SIRPα splenic DC, suggesting that these cells also endocytose apoptotic material under steady-state conditions. In addition, we have found that following i.v. transfer of allogeneic B cells, these cells are found selectively in CD4/SIRPα splenic DC. Uptake by DC is believed to occur after the B cells have been lysed by NK cells (4). Interestingly, Josien et al. have shown that freshly isolated splenic DC can lyse the NK-sensitive target cell YAC-1 (16). It is worth noting that splenic DC express the NK cell marker CD161 (NKRP-IA) and that the surface expression is not restricted to either of the subsets (18). When the two subsets of splenic DC were separated, cytolytic activity was restricted to CD4/SIRPα (17). The killing was Ca2+-independent, and did not appear to be mediated through FAS ligand, TNF-related apoptosis-inducing ligand, or TNFα. This leaves us with the intriguing possibility that one DC subset can kill host cells, engulf them, and then transport them to the T cell area of the DLN. Mechanisms regulating this process are unknown, but the expression of stress-induced markers on the target cell is one possibility. Whether killing occurs in the periphery and if that could explain the selective uptake of apoptotic epithelial cells by CD4/SIRPα iL-DC remains to be addressed.

Josien and co-workers have also characterised DC subsets in LN (17). This study defined three subsets of DC expressing αε-integrin and MHC-II. As for intestinal lymph and spleen, the CD4/SIRPα+ and subsets were identified, but the major population of LN-DC expressed very high levels of MHC-II and intermediate levels of both CD4 and SIRPα. We have also analysed MLN-DC and identified both CD4/ SIRPα+ and – DC subsets, but the MHC-IIbright DC described by Trinité et al. were much less frequent in our studies. This difference in subset ratios could be explained by heterogeneity of marker expression by DC in different LN. Trinité et al. did not specify which LN they used. We have found that expression of αε-integrin on DC differs dramatically in LN draining different types of tissue. Different levels of αε-integrin expression on subsets of DC in the same node have also been observed, CD4/SIRPα splenic DC expressing higher levels than the CD4/SIRPα+ subset (18). As purification of LN-DC by Trinité et al. was based on the mAb OX62, which recognises αε-integrin, this could bias the purification of DC from different LN as well as biasing towards selective enrichment of subsets.

In agreement with Trinité et al. we have found that thymic DC can be subdivided by their expression of SIRPα, but not by CD4. In our hands, this is the only organ where these two markers are not co-expressed by a subset of DC. This could, however, represent the subset found in spleen and LN because these cells also expressed CD5 (17).

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. LYMPH DC
  5. PERIPHERAL DC
  6. DC IN LYMPHOID TISSUES
  7. DISCUSSION
  8. CONCLUSIONS
  9. REFERENCES

The vast majority of studies of DC subsets in lymphoid tissues have used mice. Extensive studies have also been performed on subsets of DC from human blood, either freshly analysed or after expansion from monocytes by different cytokine cocktails. Plasmacytoid DC precursors have been isolated and studied in both mouse and humans, but have yet to be identified in rat. Shortman & Liu (19) have recently provided a comparative review of the properties of murine and human DC precursors and mature DC.

It is not straightforward to relate our studies and those of the Josien group to DC subsets identified in mice. Indeed it is easier to relate rat DC subsets to those in cattle and sheep (see below) than to murine DC, particularly as to date there are no reported studies of murine lymph DC.

Other problems make cross-species comparisons difficult. CD11c is the surface marker used to purify and define murine DC (there are, however, suggestions that this may not be as good a marker as is generally assumed; CD11c was raised against murine macrophages). CD11c is an unsatisfactory marker for rat DC separation (as opposed to identification) as the only mAb available in rat detects only low levels of surface expression. We do not know if this is due to low affinity or low expression on rat DC relative to mouse. In addition, most studies of differences between murine DC subsets have used CD8α for separations; CD8α is not expressed by DC in rat lymphoid organs or lymph (17, 18) (our unpublished observations). To make comparisons between surface molecule expression on DC in rat and mouse even more complicated, it was recently shown that αε-integrin, the surface molecule used to define DC in rat, is selectively expressed by the CD8α+ DC subset in mice (20). There are, however, some similarities in the surface expression of DC markers between rat and mouse. It was recently shown that about half of murine splenic DC did express CD4 (21, 22). This population had been missed for many years since it had been deleted during purification procedures because it also expresses the macrophage marker F4/80. Thus, the current definition of the major subsets of murine splenic DC includes: CD4+CD8α, CD4CD8α+ and the CD4CD8α. These three subsets have been shown to represent separate lineages (10).

If instead of using surface expression of CD8α to divide splenic DC subsets we instead use CD4, this makes the comparison somewhat easier. For example, the ratios between CD4+ and splenic DC in rats and mice would then be roughly 50:50. As mentioned above, αε-integrin expression on rat splenic DC is slightly higher on the CD4/SIRPα subset compared to the CD4/SIRPα+, which would skew the ratio if this marker were used to enrich DC from tissues (18). One difference between CD4 expression on splenic DC in mouse and iL-DC in rat is that the surface expression on murine DC is downregulated after in vitro culture but remains high on rat DC (20, 22) (our unpublished observations).

The localisation of DC subsets in situ has been studied using immunohistology. These studies showed that murine CD205 DC are found preferentially in T cell areas of spleen, LN and Peyer's Patches (23, 24). After purification from murine spleen, CD8α and DEC205 were shown to be co-expressed by the same DC subset (25), suggesting that DC in the T cell area of lymphoid organs are CD8α+. A recent study has shown that the other two subsets of murine splenic DC are also present in the T cell area, whereas a large proportion of CD8α+ DC are located in the red pulp (20). Using fluorescence microscopy we have shown that CD4/SIRPα DC are located in the T cell area of MLN and spleen, whereas CD4/SIRPα+ DC appear to be excluded from this area. These studies used the SIRPα antigen as the marker for DC subsets and it might be suggested that this marker is downregulated on DC when they enter the T cell area. However, as with CD4 expression on iL-DC we have not been able to detect downregulation of SIRPα during in vitro culture. Thus, we suggest that rat DC subsets home to different areas in lymphoid tissues. In addition, after a pro-inflammatory stimulus (i.v. LPS) we have been able to find significant numbers of CD4/SIRPα+ DC in T cell areas of nodes and spleen (Turnbull & MacPherson, in preparation).

Stimulation of murine splenic DC subsets through CD40L and subsequent analysis of cytokine production has shown that IL-12 is produced primarily by the CD4CD8α+ but that the CD4CD8α subset could produce IL-12 under optimal conditions (26). In contrast, CD4+ DC did not produce significant amounts of IL-12 under any conditions. In agreement with these findings, rat splenic DC CD4/SIRPα DC produced IL-12 after stimulation through CD40L while CD4/SIRPα+ did not (18).

As mentioned above, because CD4+ DC were not taken account of, most studies comparing the T cell priming by murine DC used CD8α to divide the subsets. Initial in vitro studies suggested that CD8α+ murine DC induced Fas/FasL-mediated death in CD4+ T cells and controlled CD8+ T cell proliferation by limiting the amount of IL-2 (27, 28). Similarly, we have shown that rat CD4/SIRPα iL-DC induce a reduced response in allogeneic T cells when compared to CD4/SIRPα+ (8). In addition, CD4/SIRPα splenic DC induce a poor proliferative response in allogeneic CD8+ T cells, whereas a strong proliferative response was observed when CD4+ T cells were used as responders (18).

Initial studies in vivo showed that when murine splenic CD8α+ DC loaded with antigen were transferred into a naïve mouse they induced a Th1-biased response in the DLN, whereas CD8α DC induced a Th2-skewed response (29, 30). Another study showed that even though the CD8α+ DC did not reach the DLN they induced priming in DLN T cells as indicated by antigen-specific T cell proliferation in vitro (31). Using freshly isolated iL-DC we have shown that CD4/SIRPα+ are much stronger in priming antigen-specific T cells in vivo compared to their CD4/SIRPα counterparts (8). Whether this reduced T cell priming capacity occurs because CD4/SIRPα iL-DC do not reach the DLN or whether they induce a tolerogenic response remains to be addressed. Studies to compare the T cell priming capacity of the two rat splenic DC subsets in vivo have not yet been performed.

As mentioned above, we have shown that rat CD4/SIRPα iL-DC selectively transport apoptotic epithelial cells to the MLN (3). Recent experiments have shown that CD8α+ DC in murine spleen, LN and liver selectively take up apoptotic material in vivo and can present peptides from the dying cells on both MHC class I and II (4). Experiments to identify a receptor that is selectively expressed by CD8α+ DC have so far been unsuccessful (32, 33). Interestingly, in one of these studies it was noted that splenic CD4CD8α DC had some capacity to phagocytose apoptotic material, which although much less than CD8α+ DC was still more than the CD4+ DC, which did not phagocytose any such material (33). In preliminary experiments we have observed selective uptake of dying cells by CD4/SIRPα splenic DC in vivo. These observations and those of Voisine et al. (18), who showed that splenic CD4/SIRPα DC contain inclusions, indicate that uptake of apoptotic material is not confined to the MLN but is rather a mechanism common to all secondary lymphoid tissues, which could be involved in sustaining tolerance to self-tissue antigens. This activity is carried out by a unique DC subpopulation in both rats and mice, and this population is in both species CD4. It is possible that the rat CD4/SIRPα DC population may include the same two populations identified in mice and that one of these subsets is superior to the other in taking up apoptotic material. We do not have a marker as yet that divides this subset; it is not CD8 as rat DC are uniformly CD8.

It is tempting to speculate that because of their similar localisation and presumed involvement in tolerance induction, the rat CD4/SIRPα DC population may equate with the murine CD8+, DEC205+ DC. However, at present we do not have any conclusive evidence that this is the case.

Data regarding DC isolated from human tissues are very limited. The few studies that are available do, meanwhile, show that there is heterogeneity in the expression of CD4, CD11b and CD11c among DC freshly isolated from spleen, thymus and tonsils (34–37). The functional significance of this heterogeneity is unknown. Studies of DC in human lymph are rare but have been performed by Brand and colleagues (38). These studies showed that CD1a+ DC were present in lymph draining the skin and that these cells had acquired antigen applied to the skin in the presence of a sensitising agent.

Many more studies have investigated human DC and their precursors in blood, but there are none in rats and very few in mice. In none of the human studies has expression of CD8 been detected. How any of the subsets of human DC identified in these studies relate to rat DC is unclear and suggestions based solely on surface marker expression are fraught with danger. Uptake of dying cells by human DC has indeed been shown (39). These DC were, however, generated from human monocytes and no studies to date have been able to assess if this capacity is confined to a particular subset of human DC.

As in rats, lymph DC have also been studied in cows, sheep and to a limited extent in humans. Bovine and ovine studies have used a model similar to the one described above for rats. Two subsets of skin L-DC (sL-DC) were identified in pseudo-afferent bovine skin lymph by their differential expression of CD5, CD11a, My-D1 and CD26 (40–42). CD5CD11alow MyD1+CD26 DC constitute about 80% of the sL-DC. The MyD1 antigen is a homologue of the rat SIRPα molecule recognised by mAb OX41 (43). Interestingly, MyD1+ sL-DC induced a strong proliferative response in both CD4+ and CD8+ T cells, while the MyD1 sL-DC only induced significant stimulation of CD4+ T cells (44). As for CD8α+ murine splenic DC or CD4/SIRPα rat iL-DC, the reduced stimulation could not be ascribed to lower surface expression of co-stimulatory molecules. Rather, proliferation appeared to be due to a lack of IL-1α production by MyD1 sL-DC. Ovine sL-DC can also be subdivided into two major subsets depending on expression of MyD1 (41). The relationship of these subsets to other L-DC is not straightforward since some ovine MyD1+ are CD4+ while some are CD8+.

Whether these two major subsets of sL-DC in sheep and cattle are equivalent to those found in rat intestinal lymph is not yet clear. Their distribution in secondary lymphoid organs and their capacity to phagocytose apoptotic material are two functional characteristics that would facilitate such a comparison.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. LYMPH DC
  5. PERIPHERAL DC
  6. DC IN LYMPHOID TISSUES
  7. DISCUSSION
  8. CONCLUSIONS
  9. REFERENCES

In all species so far examined, DC can be divided into subsets by differential expression of surface markers, and in some cases these subsets have been shown to have different properties in terms of antigen acquisition and T cell activation. In mouse and rat there is a subset that is found selectively in T cell areas of spleen and lymph nodes. In the rat this subset acquires, at least in the intestine, apoptotic cells and transports them to T cell areas of MLN. In the mouse and in the rat, these T cell area DC can acquire apoptotic cells. These DC can in mouse present antigens from the engulfed apoptotic cells on MHC class I and II, and can induce tolerance in transferred populations of TCR-transgenic T cells. It thus seems that these DC are specialised to induce tolerance to self, and by implication to harmless foreign antigens. In conditions that may reflect situations where a protective response is required, DC biology is affected in ways that are as yet poorly understood. DC, perhaps monocyte-derived, are rapidly recruited to sites of pathogen entry, exit from peripheral tissues is increased, and the distribution of subsets in secondary lymphoid organs is modulated.

Unravelling the cellular and molecular bases of the functions of DC subsets in vivo, both in the steady state and during perturbation by pathogens, will be a major challenge for immunologists in the years to come. It is becoming increasingly clear that such studies are difficult to carry out in vitro and that it is crucial to correlate all in vitro studies with studies in vivo that permit analysis under close to physiological conditions. The models developed in rats, cattle and sheep, which permit analysis of DC in the process of migration, those that are actually mediating function in lymphocyte activation, give unparalleled opportunities for solving these problems.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. LYMPH DC
  5. PERIPHERAL DC
  6. DC IN LYMPHOID TISSUES
  7. DISCUSSION
  8. CONCLUSIONS
  9. REFERENCES
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