5H). and Th2 responses in EHNA hydrochloride BAL), they also accumulate functions normally attributed to canonical CD8+ EHNA hydrochloride T cells. These hyperfunctional CD8 T cells are found to circulate peripherally as well as reside within the lymphoid tissue. Due to their unique combination of CD4 and CD8 T cell effector functions these CD4? CD8 T cells are likely able to serve as an immunophenotype capable of Th1, Tfh, and CTL functionalities, yet unable to be infected by SIV. These data demonstrate the ambiguity of CD4/CD8 expression in dictating functional capacities of T cells and suggest that accumulation of hyperfunctional CD8 T cells in EHNA hydrochloride AGM may lead to tissue-specific antiviral immune responses in lymphoid follicles which limit SIV replication in this particular anatomical niche. Introduction Simian immunodeficiency EHNA hydrochloride virus (SIV) infection of non-human primates can results in either pathogenic or non-pathogenic infection. The variability in disease outcomes depends largely on the host species. SIV infection of natural host species, including the African green monkey (AGM) and sooty mangabey (SM), results in a nonprogressive SIV infection with low levels of immune activation (1, 2). In contrast, SIV infection in nonnatural species such as the rhesus macaque, and HIV infection of humans, generally results in a pathogenic infection with high levels of immune activation, loss of CD4+ T cells, and progression to AIDS (3). Despite high plasma SIV viral loads in both non-natural and natural host species, progression to AIDS is very rare in the natural host species (4). Thus, contrasting immunological differences between the natural host and nonnatural host models may provide us with a better understanding of the mechanisms by which natural hosts have evolved to avoid disease progression. Previous studies have shown that preservation of key immunological T cell functionality in cells that are resistant to SIV-infection may be critical to the nonprogressive nature of the disease in natural host species (5C7). We have shown that multiple species of natural hosts of SIV EHNA hydrochloride have low frequencies of CD4+ T cells and high frequencies of CD4? T cells that have ARHGEF11 the capability to elicit effector functions normally attributed to CD4+ T cells (6). Recent studies have expanded on these findings demonstrating that some SIV-infected SM have very low numbers of CD4+ T cells, but high numbers of double-negative (DN) T cells that have a diverse T cell receptor repertoire and can exhibit functionality of Th1, Th2, Th17, and Tfh subsets, while maintaining proliferative capacity and resistance to SIV-infection (5). The natural host species (AGM) and (patas monkeys) that manifest low numbers of CD4+ T cells in adulthood, have large populations of CD8dim T cells that express CD8 homodimers and are distinct from the classical CD8 T cells that express the and chain of CD8 (6C10). These CD8dim T cells arise post-thymically upon transcriptional CD4 down-regulation by CD4+ T cells (7). The resulting CD8 T cells retain many functional characteristics of CD4+ T cells, including MHC class II restriction, and expression of FoxP3, CD40 ligand, IL-17, and/or IL-2 (7). However, because these T cells do not express CD4 mRNA or protein, they are resistant to SIV infection and whether this affects patterns of viral dissemination in lymphoid tissues. Here we aim to determine whether CD4 down-regulation in AGMs influences the effector functions that are typically associated with Th1, Th2, and Tfh subsets in comparison to rhesus macaque (RM) cell populations that do not down-regulate CD4. Since transcription factors involved in CD4 and CD8 lineage commitment can suppress the functional differentiation of the other cell lineage in murine models, we characterize the relative expression of ThPOK and Runx3 in the AGM T cell subsets. In murine models, a dichotomy between the transcription factors Runx3 and ThPOK dictates T cell phenotype, with ThPOK being important for both the maintenance of CD4 expression and suppression of CD8 expression while Runx3 is important for expression of CD8 and suppression of CD4 expression (18). Further, ThPOK expression in mature CD4+ T cells blocks activation of the cytolytic genes characteristic of CD8 T cells (19). Conversely, Runx3 has been shown to be able to repress ThPOK expression and play an important role in the differentiation and functionality of CD8 single positive cells (20, 21). In these models, the conditional knockout of ThPOK in.