Perhaps implying that exon 7 splicing may have been lost during evolution. Nonetheless, a D7 isoform of HLA-A*0201 showed a significantly enhanced capacity to stimulate human CD8+ T cells, suggesting that a potential loss of exon 7 splicing in humans during evolution may have had functional implications for adaptive immunity, potentially providing protection from CTLmediated autoimmunity or excessive inflammatory responses. Collectively, our data in both murine and human systems demonstrate that D7 MHC-I provides significant advantages over WT molecules for inducing superior CTL responses. Although reduced MHC-I DC surface internalization contributed to this effect, much of the increased stimulatory capacity by D7 was observed at early time points of DC-T cell contact, when WT MHC-I molecules had yet to undergo significant internalization. Confocal microscopy studies revealed that CD8+ Tofacitinib T-cell recognition of cognate antigen on peptide-pulsed APCs induced rapid clustering or ‘capping’ of WT MHC-I molecules at the site of T-cell contact that greatly limited the bio-availability of MHC-I/peptide complexes for CD8+ T cells. By contrast, exon 7- deleted MHC-I molecules showed greatly impaired T-cell induced clustering, resulting in increased MHC-I/peptide complex bioavailability and enabling APCs to stimulate more CD8+ T cells on a per-cell basis. As predicted, DCs engineered to express D7 MHC-I augmented T-cell mediated anti-tumor immunity and significantly extended mouse survival, suggesting that similar strategies could improve efficacy of human DC vaccines designed to elicit viral or tumor antigen-specific CTL responses. However, looking beyond the engineered expression of specific D7 HLA alleles in DCs, it may be more effective to target exon 7-encoded determinants pharmacologically in order to neutralize their negative impact on CTL priming, an approach that could simultaneously improve antigen presentation by all endogenouslyexpressed HLA-A, -B, and -C alleles. Many of the cellular mechanisms that govern MHC-I/peptide complex clustering, internalization and turnover in DCs remain to be elucidated. However, exon 7 does encode a highly conserved serine phosphorylation site, Ser-335, that may serve to link MHCI trafficking and antigen presentation with as-yet unidentified cellular kinases and phosphatases. In addition, exon 6 contains at least two other potential sites for post-translational modifications: a putative phosphorylation site at Tyr-320, and a highly-conserved ubiquitination site at Lys-316.