Repertoire-determining role of peptide in the positive selection of CD8+ T cells

A single-amino-acid variant of the H60 CD8 epitope generates specific immunity with diverse TCR recruitment

TCR of CD8 T cells recognizes peptides of 8-9 amino acids in length (epitope) complexed with MHC class I. Peptide ligands differing from an epitope by one or two amino acids are thought to modulate the immune response specific to that epitope. H60 is a minor histocompatibility antigen for which the specific CD8 T-cell response dominates during alloresponse after MHC-matched allogeneic transplantation. In the present study, we developed a transgenic mouse (designated H60H Tg) expressing a variant of H60, designated H60H, in which the arginine residue at position 4 of the H60 epitope sequence (LTFNYRNL) is replaced by a histidine residue (LTFHYRNL). Immunization of female C57BL/6 mice with splenocytes from male H60H Tg induced a CD8 T cell primary response and memory response after re-challenge. The response was CD4 help-dependent, demonstrating the potency of H60H as a cellular antigen. The response induced by the H60H cellular antigen was comparable to that induced by H60 in its peak magnitude and overall immune kinetics. H60H challenge recruited broadly diverse TCRs to the specific response, shaping a TCR repertoire different from that of the natural H60 epitope. However, some of the TCRs did overlap between the H60H- and H60-specific CD8 T cells, suggesting that H60H might modulate the H60-specific response. These results may provide a basis for the modulation of the H60-specific CD8 T-cell response.

Grb2 functions at the top of the T-cell antigen receptor-induced tyrosine kinase cascade to control thymic selection

Grb2 is an adaptor molecule that mediates Ras-MAPK activation induced by various receptors. Here we show that conditional ablation of Grb2 in thymocytes severely impairs both thymic positive and negative selections. Strikingly, the mutation attenuates T-cell antigen receptor (TCR) proximal signaling, including tyrosine phosphorylation of multiple signaling proteins and Ca(2+) influx. The defective TCR signaling can be attributed to a marked impairment in Lck activation. Ectopic expression of a mutant Grb2 composed of the central SH2 and the C-terminal SH3 domains in Grb2(-/-) thymocytes fully restores thymocyte development. Thus, Grb2 plays a pivotal role in both thymic positive and negative selection. It amplifies TCR signaling at the top end of the tyrosine phosphorylation cascade via a scaffolding function.

Identification of tumor antigen-specific cytotoxic T lymphocytes cross-recognizing allogeneic major histocompatibility class I molecules

Adoptive immunotherapy of cancer utilizes tumor antigen-specific cytotoxic T lymphocytes (CTL) as mediators of a targeted anti-tumor effect. In this study, we show that such CTL can be able to cross-recognize allogeneic major histocompatibility complex (MHC) molecules in a phenomenon of molecular mimicry. A self histo-leukocyte antigen (HLA) A*0201-restricted CTL specific for peptide MT27-35 from the human differentiation antigen Melan-A/MART-1 was shown to cross-recognize allogeneic A*0220 molecules which differ from syngeneic A*0201 for a single amino acid substitution at position 66 of the antigen-binding groove. A*0220 molecules were recognized on a variety of human cells of different histological origin but not on COS-7 cells. A second self-A*0201-restricted CTL, specific for peptide D10/6-271 encoded by the tumor-specific DAM-gene family, was shown to cross-recognize allogeneic B*3701 molecules which differ from syngeneic A*0201 by 32 amino acids in the peptide antigen-binding cleft. B*3701 molecules were recognized on a variety of cell types including COS-7 cells. These data raise new safety issues for clinical trials of cancer immunotherapy using adoptive transfer of in vitro generated, allogeneic CTL with specific anti-tumor activity.

Potent effects of low levels of MHC class II-associated invariant chain on CD4+ T cell development

Invariant chain (Ii)-negative mice exhibit defects in MHC class II assembly and transport that results in reduced levels of surface class II, altered antigen presentation, and inefficient positive selection of CD4+ T cells. Many CD4+ T cells that do mature in Ii-negative mice express a cell surface phenotype consistent with aberrant positive selection or peripheral activation. Reconstitution of these mice with low levels of either the p31 or p41 form of Ii does not restore transport of the bulk of class II or class II surface expression, but surprisingly does restore positive selection as measured by numbers and surface phenotype of CD4+ T cells. Thus, an Ii-dependent process, independent of effects on class II surface density, appears to be required for normal positive selection of CD4+ T cells.