The CD8αα homodimer, a ligand for the non-classical major histocompatibility complex (MHC) molecule

thymic leukaemia antigen,51 is transiently expressed on CD8αβ+50 T cells that down-regulated the CD8β chain. Studies performed on human blood samples identified CD8αα+ T cells as a particular memory T-cell subset47,48 which is stable over time52 and enriched in antigen-specific T cells. Our data showed that CD8αα+ T cells are not only present in NHPs, MAPK Inhibitor Library purchase but are also present at higher frequency, in the peripheral circulation of NHPs, and that in HDs and NHPs CD8αα+ T cells were enriched in differentiated T cells compared with CD8αβ+ T cells. The NHP CD8αα+ T cells may therefore also represent a memory T-cell subsets for long-lived antigen-specific immune responses:53 we have previously shown that NHP CD8αα+ T cells, and not CD8αβ+ T cells specifically proliferate in response to molecularly defined Mycobacterium tuberculosis antigens.53 Down-regulation of the CD8β chain may represent a mechanism that lowers the avidity of the TCR to its MHC–peptide CP-673451 mouse ligand to secure long-term immune cell memory limiting T-cell activation54 and the risk of activation-induced apoptosis.55,56. Two additional T-cell compartments were present in HDs and at a higher frequency in NHPs: CD4+ CD8αα+ and CD4+ CD8αβ+ T cells as reported previously.57–59 Their frequency appeared to be higher in female rhesus monkeys.20

CD4+ CD8+ T cells stained positive for the degranulation marker CD107a. In contrast to a previous report,59 CD4+ CD8αα+ and CD4+ CD8αβ+ T cells in NHPs showed similar frequencies and their maturation/differentiation marker profile reflected the phenotype of the ‘conventional’ CD4+ CD8– T

Etomidate cells. We postulate that CD4+ CD8+ T cells represent a specialized compartment of CD4+ T cells formed during the different stages of T-cell differentiation, characterized by CD8 expression. Because the CD4+ CD8+ T cells were endowed with effector capacity (CD107a expression) (model Fig. 7); it could be that CD4+ CD8− T cells represent a CD4+ T-cell compartment capable of lysing target cells, the co-expression of CD8 enables intracellular calcium levels to be increased, enhances cytotoxicity and may prevent apoptosis60 upon binding to MHC class I molecules. To examine the role of CD4+ CD8+ T cells, we evaluated IL-17 production in PBMCs from HDs and NHPs in the presence IL-23 and IL-1β.61 Only data from HDs could be analysed because of the low number of IL-17-positive events in NHP PBMCs. CD4+ CD8+ T cells showed a higher, and CD8αα+ T cells a comparable, frequency of IL-17 production, yet a different profile (more polyfunctional IL-17+ TNF-α+ IFN-γ+) as compared with CD4+ (CD8−) T cells. These data support the notion that CD4+ CD8+ T cells appear to represent a distinct CD4+ T-cell memory compartment, in part characterized by IL-17 production.