Peptide dependency of alloreactive CD4+ T cell responses

Revisiting nonclassical HLA II functions in antigen presentation: Peptide editing and its modulation

The nonclassical major histocompatibility complex of class II molecules (ncMHCII) HLA-DM (DM) and HLA-DO (DO) feature essential functions for the selection of the peptides that are displayed by classical MHCII proteins (MHCII) for CD4⁺ T_h cell surveillance. Thus, although the binding groove of classical MHCII dictates the main features of the peptides displayed, ncMHCII function defines the preferential loading of peptides from specific cellular compartments and the extent to which they are presented. DM acts as a chaperone for classical MHCII molecules facilitating peptide exchange and thereby favoring the binding of peptide-MHCII complexes of high kinetic stability mostly in late endosomal compartments. DO on the other hand binds to DM blocking its peptide-editing function in B cells and thymic epithelial cells, limiting DM activity in these cellular subsets. DM and DO distinct expression patterns therefore define specific antigen presentation profiles that select unique peptide pools for each set of antigen presenting cell. We have come a long way understanding the mechanistic underpinnings of such distinct editing profiles and start to grasp the implications for ncMHCII biological function. DM acts as filter for the selection of immunodominant, pathogen-derived epitopes while DO blocks DM activity under certain physiological conditions to promote tolerance to self. Interestingly, recent findings have shown that the unexplored and neglected ncMHCII genetic diversity modulates retroviral infection in mouse, and affects human ncMHCII function. This review aims at highlighting the importance of ncMHCII function for CD4⁺ T_h cell responses while integrating and evaluating what could be the impact of distinct editing profiles because of natural genetic variations.

Endogenous-peptide-dependent alloreactivity: new scientific insights and clinical implications

T-cell alloreactivity is generated via immune responsiveness directed against allogeneic (allo) human leucocyte antigen (HLA) molecules. Whilst the alloresponse is of extraordinary potency and frequency, it has often been assumed to be less peptide-specific than conventional T-cell reactivity. Recently, several human studies have shown that both alloreactive CD8(+) and CD4(+) T cells exhibit exquisite allo-HLA and endogenous peptide specificity that has also underpinned tissue-specific allorecognition. In this review, we summarize former and recent scientific evidence in support of endogenous peptide (self-peptide)-dependence of T-cell alloreactivity. The clinical implications of these findings will be discussed in the context of both solid organ transplantation and haematopoietic stem cell transplantation (HSCT). Insights into the understanding of the molecular basis of T-cell allorecognition will probably translate into improved allograft survival outcomes, lower frequencies of graft vs host disease and could potentially be exploited for selective graft vs leukaemia effect to improve clinical outcomes following HSCT.

A new T-cell receptor transgenic model of the CD4+ direct pathway: level of priming determines acute versus chronic rejection

BACKGROUND

T-cell receptor transgenic (TCR-tg) mouse models with direct CD4 alloreactivity will help elucidate mechanisms of transplant rejection and tolerance in vivo. Although such models exist, they are limited by unusual strain combinations or are based on model antigens.

METHODS

A TCR-tg mouse with direct CD4 specificity in the widely used BALB/c donor --> C57BL/6 host strain combination was created. This TCR-tg mouse, named 4C, was selected for reactivity against BALB/c dendritic cells in order to model early priming events after transplantation. The response of 4C T cells to skin and heart transplants were characterized.

RESULTS

The alloantigen is restricted by I-A and appears to be widely distributed in mouse tissues. 4C T cells are able to acutely reject skin but not heart allografts. Paradoxically, heart grafts elicited a stronger proliferation and effector function of TCR-tg T cells than skin grafts. 4C T cells caused cardiac allograft vasculopathy in the absence of other T cells and alloantibodies, suggesting a role for the direct pathway in chronic rejection. Augmentation of priming with an infusion of donor-derived dendritic cells resulted in acute heart allograft rejection by 4C T cells, demonstrating that the level of priming can play a role in determining acute versus chronic rejection by the CD4 direct pathway.

CONCLUSIONS

Rejection of a graft by the direct CD4 pathway is determined by graft susceptibility to rejection, as well as the degree of T-cell priming caused by the graft. Grafts that are not acutely rejected can develop transplant vasculopathy mediated by the direct CD4 T cells.

Alloreactive T cells respond specifically to multiple distinct peptide-MHC complexes

The molecular basis underlying the specificity of alloreactive T cells for peptide-major histocompatibility complex ligands has been elusive. Here we describe a screen of 60 I-E(k)-alloreactive T cells and 83 naturally processed peptides that identified 9 reactive T cells. Three of the T cells responded to multiple, distinct peptides that shared no sequence homology. These T cells recognized each peptide-major histocompatibility complex ligand specifically and used a distinct constellation of I-E(k) contact residues for each interaction. Our studies show that alloreactive T cells have a 'germline-encoded' capacity to recognize multiple, distinct ligands and thus show 'polyspecificity', not degeneracy. Our findings help to explain the high frequency of alloreactive T cells and provide insight into the nature of T cell specificity.

T cell recognition of an engineered MHC class I molecule: implications for peptide-independent alloreactivity

Previously, we described H-2K(bW9) (K(bW9)), an engineered variant of the murine MHC class I molecule H-2K(b) (K(b)), devoid of the central anchor ("C") pocket owing to a point mutation on the floor of the peptide binding site; this substitution drastically altered selection of bound peptides, such that the peptide repertoires of K(b) and K(bW9) are largely nonoverlapping in vivo. On the basis of these observations, we used K(bW9) and K(b) to revisit the role of peptides in alloreactive T cell recognition. We first compared Ab and TCR recognition of K(bW9) and K(b). Six of six K(b)-specific mAbs, directed against different parts of the molecule, recognized K(bW9) well, albeit at different levels than K(b). Furthermore, K(bW9) readily served as a restriction element for a peptide-specific syngeneic CTL response. Therefore, K(bW9) mutation did not result in gross distortions of the TCR-interacting surface of class I, which was comparable between K(b) and K(bW9). Interestingly, when K(bW9) was used to stimulate allogeneic T cells, it induced an infrequent CTL population that cross-reacted against K(b) and was specific for peptide-independent MHC epitopes. By contrast, K(b)-induced alloreactive CTLs recognized K(b) in a peptide-specific manner, did not cross-react on K(bW9), and were present at much higher frequencies than those induced by K(bW9). Thus, induction of rare peptide-independent CTLs depended on unique structural features of K(bW9), likely due to the elevated floor of the peptide-binding groove and the consequent protruding position of the peptide. These results shed new light on the relationship between TCR and peptide-MHC complex in peptide-independent allorecognition.

Comparison of Abeta(b-/-), H2-DM(-), and CIITA(-/-) in second-set skin allograft rejection

BACKGROUND

Responses against donor MHC antigens are the major contributor to allograft rejection. Currently, it is unclear whether both direct and indirect recognition pathways are necessary and/or sufficient for allograft rejection. Previously, we found donor MHC class II and H2-DM to have dramatic effects on cardiac allograft survival.

METHODS

Here, we used H2-DM(-) mice, which express CLIP-MHC class II complexes, and CIITA(-/-) mice, which lack all class II proteins, to examine the role of direct and indirect recognition on skin allograft rejection. Recipients were primed with donor cultured keratinocytes and later tested for accelerated memory response by challenge with full-thickness tail skin grafts.

RESULTS

As previously reported, Abeta(b-/-) grafts survived longer than wild-type grafts, while H2-DM(-) grafts were rejected as rapidly as wild-type grafts. Skin grafts deficient for both beta2m and H2-DM survived longer than grafts lacking only H2-DM, but not as long as Abeta(b-/-) grafts. Additionally, CIITA(-/-) grafts survived as long as Abeta(b-/-) grafts.

CONCLUSIONS

The delayed rejection of Abeta(b-/-) compared to H2-DM(-) suggests that indirect recognition of surface-expressed donor MHC class II is sufficient to mediate rapid skin allograft rejection. The equivalent survival of CIITA(-/-) and Abeta(b-/-) grafts suggests that indirect presentation of donor class II molecules (Aalpha or Ebeta) present in Abeta(b-/-) but not CIITA(-/-) mice does not contribute to graft rejection. These results reveal a modest role for surface-expressed donor class II in primed keratinocyte rejection, but also reveal a dramatic contrast to the cardiac allograft system and indicate tissue/organ-specific mechanisms of rejection.

Low level lead exposure in vitro stimulates the proliferation and expansion of alloantigen-reactive CD4(high) T cells

T cells are believed to be critical functional targets of Pb immunotoxicity. In this study, low concentrations of lead (i.e., as low as 0.1 microM approximately 2 microg/dl) were found to markedly enhance the allogeneic mixed lymphocyte reaction-an assay of CD4(+) T cell responsiveness. Cell cycle analysis of cells recovered from allogeneic mixed lymphocyte cultures revealed that Pb stimulated a substantial increase in the proportion of cycling alloreactive CD4(+) T cells. The enhanced alloproliferative response was characterized by an increased population of lymphoblasts expressing heightened cell surface expression of CD4 (i.e., CD4(high) cells). Successive rounds of cell division were monitored using the cell division dye 5- (and 6)-carboxyfluorecein diacetate succinimyl ester and it was determined that the CD4(high) subpopulation comprised the expanding alloreactive T cells, which ultimately took on the phenotype of memory/effector T cells (i.e., CD44(high), CD45RB(low), CD69(high), and CD162(high)). Enhancement of T cell proliferation by lead was selective for responsiveness to alloantigen, as lead had no effect on T cell proliferation induced by mitogens or superantigen, processes that unlike alloreactivity are not dependent on antigen presentation. Collectively, these data suggest that Pb enhances alloantigen-specific T cell proliferation through an indirect mechanism involving altered antigen processing/presentation, resulting in marked clonal expansion or repertoire expansion of alloreactive T cell clones. Consistent with this suggestion was the finding that a single exposure to Pb during alloantigen priming elicited a population of CD4(+) T cells that was hyperresponsive to further alloantigen stimulation and neither lead dependent nor lead responsive.

Activation of a T cell hybridoma by an alloligand results in differential effects on IL-2 secretion and activation-induced cell death

The molecular nature of the interaction of T cell receptors (TCR) with alloligands is not well understood. Although a role for groove-bound peptide(s) has been clearly demonstrated for major histocompatibility complex (MHC) class I alloreactivity, this has not been established for MHC class II-induced alloresponses. In the present study, we have analyzed the interaction of a nominal peptide-self MHC complex and of an alloligand with their cognate TCR (1934.4 TCR for autoantigen recognition and qCII85.33 TCR for allorecognition). Our results demonstrate that 1934.4 TCR recognition of the N-terminal epitope of myelin basic protein (Ac1-11, Ac=acetylated at position 1) complexed with the MHC class II molecule I-A(u) involves contacts with both chains of the MHC molecule. In contrast, qCII85.33 TCR recognition of an allopeptide:I-A(u) complex appears to predominantly involve the beta chain of the MHC molecule. Thus, the two TCR appear to have different footprints on the I-A(u) molecules. Unexpectedly, this differential involvement of the two chains of the I-A(u) molecule affects activation induced cell death, with allostimulation resulting in poor induction of FasL expression and relatively low levels of apoptosis. Significantly, stimulation of cognate T cells with alloantigen or autoantigen results in similar levels of IL-2 secretion. The reduced apoptosis of T cells in response to allostimulation may be one of the mechanisms that favors the expansion of a relatively large repertoire of alloreactive T cells.

The alloreactive and self-restricted CD4+ T cell response directed against a single MHC class II/peptide combination

The cellular basis for allograft rejection derives from the strong T cell response to cells bearing foreign MHC. While it was originally assumed that alloreactive T cells focus their recognition on the polymorphic residues that differ between syngeneic and allogeneic MHC molecules, studies with MHC class I-restricted CTL have shown that MHC-bound peptides play a critical role in allorecognition. It has been suggested that alloreactive T cells depend more strongly on interactions with the MHC molecule than with the associated peptide, but there is little evidence to support this idea. Here we have studied the alloreactive and self-restricted response directed against the class II H2-Ab molecule bound with a single peptide, Ep, derived from the H2-Ealpha chain. This MHC class II-peptide combination was a poor target and stimulator of alloreactive CD4+ T cell responses, indicating that MHC-bound peptides are as important for alloreactive CD4+ T cells as they are for alloreactive CTL. We also generated alloreactive T cells with exquisite specificity for the Ab/Ep complex, and compared their reactivity with self-restricted T cells specific for the same Ab/Ep complex. Our results showed that peptide-specific alloreactive T cells, as compared with self-restricted T cells, were more sensitive to peptide stimulation, but equally sensitive to amino acid substitutions in the peptide. These findings indicate that alloreactive and self-restricted T cells interact similarly with their MHC/peptide ligand.

H2-DMalpha(-/-) mice show the importance of major histocompatibility complex-bound peptide in cardiac allograft rejection

The role played by antigenic peptides bound to major histocompatibility complex (MHC) molecules is evaluated with H2-DMalpha(-/)- mice. These mice have predominantly class II-associated invariant chain peptide (CLIP)-, not antigenic peptide-bound, MHC class II. H2-DMalpha(-/)- donor heart grafts survived three times longer than wild-type grafts and slightly longer than I-A(beta)(b)-(/)- grafts. Proliferative T cell response was absent, and cytolytic response was reduced against the H2-DMalpha(-/)- grafts in vivo. Residual cytolytic T cell and antibody responses against intact MHC class I lead to eventual rejection. Removal of both H2-DMalpha and beta2-microglobulin (beta2m) in cardiac grafts lead to greater (8-10 times) graft survival, whereas removal of beta2m alone did not have any effect. These results demonstrate the significance of peptide rather than just allogeneic MHC, in eliciting graft rejection.

Quantitative analysis of peptide-MHC class II interaction

The tremendous progress in the field of basic immunology and immunochemistry made in the last decade has significantly advanced our understanding of antigen processing and presentation by MHC class I and II proteins. In this review different techniques to study peptide interaction with MHC class II molecules are summarized and their impact on the elucidation of quantitative parameters, like affinities or kinetic data, is discussed. A recently introduced method based on synthetic combinatorial peptide libraries allows to quantify the binding contribution of each amino acid residue in a class II ligand and is presented in more detail. As this knowledge is fundamental for current investigations in modern medicine, e.g. for novel immune system based therapy concepts, further aspects like the design of new high affinity MHC class II ligands and the prediction of peptide antigens are discussed.