Cutting Edge: CD4 Is the Receptor for the Tick Saliva Immunosuppressor, Salp15

Abstract

Salp15 is an Ixodes scapularis salivary protein that inhibits CD4+ T cell activation through the repression of TCR ligation-triggered calcium fluxes and IL-2 production. We show in this study that Salp15 binds specifically to the CD4 coreceptor on mammalian host T cells. Salp15 specifically associates through its C-terminal residues with the outermost two extracellular domains of CD4. Upon binding to CD4, Salp15 inhibits the subsequent TCR ligation-induced T cell signaling at the earliest steps including tyrosine phosphorylation of the Src kinase Lck, downstream effector proteins, and lipid raft reorganization. These results provide a molecular basis to understanding the immunosuppressive activity of Salp15 and its specificity for CD4+ T cells.

The Ixodes scapularis salivary protein, salp15, prevents the association of HIV-1 gp120 and CD4

“…The C-terminal portion of Salp15 (amino acids 95-114 of the mature protein named P11, with the sequence GPNGQTCAEKNKCVGHIPGC - [17]) mediates its interaction with sCD4 [18] and recapitulates the immunosuppressive activity of Salp15 [26]. We thus tested the ability of purified P11 to compete CD4-gp120 binding.…”

“…To test whether Salp15 is able to bind to gp120, we performed microtiter binding assays using Salp15 and synthetic 20 amino acid-long peptides with 10 amino acids overlaps encompassing the entire Salp15 protein sequence (P1-P11) [18]. Plate-bound Salp15 showed a dosedependent binding to soluble HRP-gp120 ( Figure 3A).…”

“…Plate-bound Salp15 showed a dosedependent binding to soluble HRP-gp120 ( Figure 3A). Only the carboxyl terminal peptide (P11) of Salp15 [18] was able to bind to gp120 ( Figure 3B). We could also compete the interaction of P11 with gp120 by adding increasing concentrations of free Salp15 ( Figure 3C), suggesting a specific interaction.…”

“…Recombinant HIV-1 IIIB HRP-conjugated gp120 and sCD4 (domains D1D2) were purchased from Immunodiagnostics, Inc. (Woburn, MA). The Salp15 overlapping peptides (P1-P11) were previously described [18]. The peptide CD4M9 (CNLARCQLRCKSLGLLGKCAGSFCACGP) [19,20] …”

“…Inhibition mediated by Salp15 results from the repression of calcium fluxes triggered by T cell antigen receptor (TCR) ligation and a subsequent reduction in interleukin (IL)-2 production. Salp15 binds to the most extracellular domains (D1-D2) of the T cell co-receptor CD4 in both mouse and human cells [18] in a region that may overlap with the binding residues of gp120.…”