Functional analysis of the salivary protein, Salp15
The interaction between Ixodid ticks and their mammalian hosts is a complex relationship. While the mammalian host tries to avoid the completion of the feeding process, the tick has devised strategies to counteract these attempts. Tick saliva contains a vast array of pharmacological activities that presumably aid the tick to evade host responses, including anti-complement, oxidative and innate and adaptive immune responses. The characterization of these activities has gained momentum in the last several years. One of the best-studied activities present in tick saliva corresponds to the antigen known as Salp15, which inhibits TCR-mediated CD4+ T cell activation and IL-2 production.
This study identifies CD4 as the specific receptor for Salp15 on T cells. A direct association occurs between CD4 and the C-terminal amino acids of Salp15, which allows Salp15 to act at the very beginning of TCR ligation-induced signaling cascades. Salp15 prevents the activation of Lck upon TCR engagement and the formation of lipid rafts and actin polymerization. Salp15 also affects tyrosine phosphorylation of several early signal components downstream of Lck, including Zap70, Lat, and PLCγ1. This study also demonstrates that the peptide that mediates the interaction of Salp15 with CD4, P11, is able to recapitulate the immunosuppressive activity of the whole saliva protein. These results clarify the molecular mechanisms of action of Salp15 on T cells and suggest that binding to CD4 is sufficient to elicit its immunosuppressive activity.
The differentiation of Th17 cells requires the concerted action of IL-6 and TGFβ. However, the exact contribution of each cytokine has not been elucidated. This study also provides evidence indicating that the role of TGFβ during the differentiation of CD4+ T cells is exclusively the repression of IL-2 production, which has been shown to counteract the generation of these effector cells. The inhibition of IL-2 during the differentiation of CD4+ T cells mediated by Salp15 and the specific IP 3 receptor inhibitor 2-APB resulted in Th17 differentiation in vitro. Furthermore, the treatment of PLP139-151-immunized mice with Salp15 also resulted in increased differentiation of Th17 cells in the central nervous system and augmented pathology. Our results demonstrate that the role played by TFGβ is circumscribed to the inhibition of IL-2 during the differentiation of CD4+ T cells.
Finally, this study shows that Salp15 inhibits the interaction between HIV-1 gp120 and CD4. Furthermore, Salp15 prevents the formation of syncitia of HL2/3 (a stable HeLa cell line expressing the envelop protein) and CD4-expressing cells. Salp15 prevented gp120-CD4 interaction at least partially through its direct interaction with the glycoprotein. A phage display library screen provided the interacting residues in the C1 domain of gp120. These results provide a potential basis to define exposed gp120 epitopes for the generation of neutralizing vaccines against HIV.