Self-tolerance and the composition of T cell repertoire

Increased sensitivity to antigen in high avidity CD8(+) T cells results from augmented membrane proximal T-cell receptor signal transduction

The functional avidity of a cytotoxic T lymphocyte (CTL) is known to be a critical determinant of the efficacy with which it clears pathogens. High avidity cells, which are by definition highly sensitive to peptide antigen, are superior for elimination of viruses and tumours. Our studies have established the ability of T cells to undergo avidity modulation as a result of antigen encounter. High and low avidity cells established in this manner exhibit significant differences in the amount of peptide required to elicit effector function. However, how signalling is regulated in these cells as it relates to the control of peptide sensitivity remains to be defined. To address this question, we compared T-cell receptor (TCR) signal transduction events in high and low avidity CTL generated from OT-I(rag2-) TCR transgenic mice. Our data suggest that divergent signalling is initiated at the TCR-associated CD3ζ, with low avidity CTL requiring higher amounts of pMHC to achieve threshold levels of phosphorylated CD3ζ compared with high avidity CTL. Further, this difference is transduced further downstream to mitogen-activated protein kinase and Ca(2+) signalling pathways. These results suggest that regulated control of the initiation of TCR signalling in high versus low avidity cells determines the amount of peptide required for T-cell activation.

Highly diverged MHC class I mismatches are acceptable for haematopoietic stem cell transplantation

A fully major histocompatilbility complex (MHC) matched donor is not available for the majority of patients in need of a haematopoietic stem cell transplantation (SCT), which illustrates the need for a tool to define acceptable MHC disparities. Previously, we noticed that a variety of single MHC class I mismatched allogeneic donor-recipient pairs did not elicit an allogeneic cytotoxic-lymphocyte (CTL) response in vitro if the MHC amino-acid sequences had five or more differences in the alpha-helices plus five or more differences in the beta-sheet (> or =5alpha5beta) (7). To address the clinical relevance of this observation, we analysed CTL precursor (CTLp) assay outcome and SCT outcome in 53 Dutch recipients of a single MHC class I mismatched graft from an unrelated donor. Overall patient survival was 44% after 4 years. In multivariate analysis, recipients of a > or =5alpha5beta mismatched graft with negative CTLp frequencies in vitro before transplantation demonstrated superior survival: survival at 4 years was 80% as compared to 47% in recipients of other mismatched grafts with negative CTLp frequencies (hazard ratio=0.131; 95% CI=(0.03-0.61); P=0.009). This option of acceptable mismatches may enlarge the pool of potentially acceptable stem cell donors.

Visualization of p53(264-272)/HLA-A*0201 complexes naturally presented on tumor cell surface by a multimeric soluble single-chain T cell receptor

Intracellular Ags are processed into small peptides that are presented on cell surfaces in the context of HLA class I molecules. These peptides are recognized by TCRs displayed by CD8+ T lymphocytes (T cells). To date, direct identification and quantitation of these peptides has relied primarily on mass spectrometry analysis, which is expensive and requires large quantities of diseased tissues to obtain useful results. Here we demonstrate that multimerization of a soluble single-chain TCR (scTCR), recognizing a peptide from p53 presented in the context of HLA-A2.1, could be used to directly visualize and quantitate peptide/MHC complexes on unmanipulated human tumor cells. Tumor cells displaying as few as 500 peptide/MHC complexes were readily detectable by flow cytometry. The scTCR/multimers exhibited exquisite recognition capability and could distinguish peptides differing in as little as a single amino acid. We also demonstrate that scTCR/multimers could specifically stain human tumors generated in mice, as well as tumors obtained from patient biopsies. Thus, scTCR/multimers represent a novel class of immunostaining reagents that could be used to validate, quantitate, or monitor epitope presentation by cancer cells.

Allogeneic MHC Class I Molecules With Numerous Sequence Differences Do Not Elicit a CTL Response

CD8+ T cell-mediated alloreactivity is generally believed to involve recognition of the alpha1/alpha2 domains of donor-type class I MHC molecules as well as the peptides they bind. Using the CTLp assay outcome as a parameter for the induction of alloreactivity, we have retrospectively surveyed 80 haematopoietic stem cell donor/patient pairs that feature a range of allelic differences at single HLA-A, -B, and -C loci in an attempt to probe the predictive value of such mismatches. In contrast to the expectation that greater degree of allelic disparity would lead to more alloreactivity, we found that in a substantial number of cases, class I MHC molecules with numerous sequence differences did not elicit an allogeneic CTL response. We propose that in generating a T cell repertoire with a sufficiently narrow responsive for self-MHC, positive thymic selection limits the capacity to recognize allogeneic MHC molecules whose structure and sequence have diverged extensively. These findings are important for donor and patient MHC matching strategies and our understanding of T cell-MHC interaction after haematopoietic stem cell transplantation.

CD8+ cytotoxic T lymphocytes isolated from allogeneic healthy donors recognize HLA class Ia/Ib-associated renal carcinoma antigens with ubiquitous or restricted tissue expression

Allogeneic hematopoietic stem cell transplantation can induce considerable tumor remissions in metastatic renal-cell carcinoma (RCC) patients. The precise effector mechanisms mediating these graft-versus-tumor reactions are unknown. We studied RCC-directed CD8(+) T-cell responses in blood lymphocytes of healthy individuals matched with established RCC cell lines for HLA-class I. In 21 of 22 allogeneic mixed lymphocyte/tumor-cell cultures (MLTCs), RCC-reactive cytotoxic T-lymphocytes (CTLs) were readily obtained. From MLTCs, 121 CD8(+) CTL clones with memory phenotype were isolated. Their anti-RCC reactivity was restricted by multiple classical HLA-Ia molecules, in particular by HLA-A2, -A3, -B7, -B44, -Cw7, and by a nonclassical HLA-Ib determinant. Extensive cross-reactivity analyses on a broad target panel identified CTLs that recognize antigens with expression restricted to renal tissue or to renal and colon tumors. Other CTLs were directed against antigens with broader tissue distribution being expressed in various epithelial and nonepithelial tumors or, additionally, in hematopoietic cells. With microcapillary liquid chromatography and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF)/TOF mass spectrometry, we identified the HLA-A*0301-associated nonpolymorphic peptide KLPNSVLGR encoded by the ubiquitously expressed Eps15 homology domain-containing 2 gene as a CTL target. Defining human RCC antigens recognized by alloreactive CTLs may allow to improve the specificity and efficiency of allogeneic cell therapy (eg, specific donor-lymphocyte infusions or vaccination) in metastatic RCC patients.

TCRs with high affinity for foreign pMHC show self-reactivity

T cells with high-affinity T cell receptors (TCRs) for a foreign peptide-major histocompatibility complex (pMHC) appear to be negatively selected, even though they have never seen the foreign antigen. To examine how this process operates, we used in vitro yeast display to isolate high-affinity TCRs from the T cell clone 2C. The TCRs showed fast on-rates, which were consistent with reduced CDR (complementarity determining region) flexibility, and cross-reactivity with other cognate pMHCs. T cell hybridomas transfected with a high-affinity TCR were stimulated by endogenous self-pMHC, which suggested that T cells bearing the TCR would be negatively selected. The immune system appears to maintain a repertoire of flexible, low-affinity TCRs at the expense of more effective high-affinity TCRs.

The natural killer cell -- friend or foe in autoimmune disease?

Autoimmune diseases are chronic conditions resulting from a loss of immunological tolerance to self-antigens. Recent observations have supported an ever-broader role for innate immune responses in directing and regulating adaptive immunity, including responses to self. This review summarizes recent findings supporting important functions of natural killer (NK) cells in regulating autoimmunity. A close survey of the current literature reveals multiple steps where NK cells can regulate inflammation and intervene in loss of self-tolerance. Importantly, the findings also caution against inferring a similar role for NK cells in all autoimmune phenomena or during separate stages of the same disease. Indeed, NK cells may have different influences during the priming and the effector phases of disease. Hence, an increased understanding of the involvement of NK cells in inflammation and infection should provide new insights into the pathogenesis of autoimmune disease.

Expression of stress-induced MHC class I related chain molecules on human melanoma

Cellular immune responses to melanoma are tightly regulated and include specific T cell responses to self antigens such as Mart-1 and gp100. Thus, additional signals apart from those mediated by the T cell receptor are needed to ensure T cell activation. Recently, the stress inducible major histocompatibility complex molecules, MHC class I related chain, were identified as an activator of both natural killer and T cells via interaction with their receptor NKG2D. Herein, we report the expression of MIC in 31 of 40 primary cutaneous melanomas and in 13 of 20 metastatic lesions. Moreover, lymphocytes infiltrating the tumor were found to express NKG2D. Detailed analysis identified both CD3+ T cells as well as CD56+ natural killer cells contributing to this NKG2D+ tumor infiltrating lymphocyte population present.

Tumour immunotherapy: new tools, new treatment modalities and new T-cell antigens

Tumour immunology has seen many exciting developments in the last few years. In addition to tumour antigens that are defined by antitumour T- and B-cell responses in patients, the human telomerase reverse transcriptase has been identified by 'reverse immunology' as the first truly universal tumour antigen. Molecular remission has been associated with a cancer vaccine that targets the clonal idiotype of B-cell malignancies, and sophisticated cellular vaccines (including fusions of tumour cells and antigen-presenting cells) have demonstrated promising results. Moreover, our capabilities of measuring immunity have been significantly enhanced by novel technology, such as major histocompatibility complex (MHC)-peptide tetramers and ELISPOT analysis. We are now capable of tracking antigen-specific T cells at a single cell level. This review will analyse recent developments and highlight some important issues that need to be addressed in the future.