Engagement of IL-1 receptor accessory protein (IL-1RAcP) with the monoclonal antibody AY19 provides co-activating signals and prolongs the CD2-induced proliferation of peripheral blood lymphocytes.

IL-1 receptor accessory protein (IL-1RAcP) is the second subunit required to form a functional receptor complex for IL-1α and β, IL-1F6, IL-1F8, IL1-F9 and IL-33. While it does not directly interact with the cytokines, IL-1RAcP is necessary to mediate signal transduction. We previously reported a monoclonal antibody with an unknown specificity, termed AY19, that was capable to induce a significant increase in the size of CFU-GM colonies when added to cultures of human cord blood CD34(+) hematopoietic progenitors. Here we demonstrate that AY19 mAb recognizes IL1-RAcP. We show that this adaptor molecule is significantly present on peripheral blood monocytes and lymphocytes including CD4(+) and CD8(+) T lymphocytes, B and NK cells. Interestingly, its expression is found increased on CD127(low)CD4(+)CD25(high) T cells when compared to CD127(low)CD4(+)CD25(-) T cell subset, suggesting that the level of IL-1RAcP membrane expression could allow to distinguish within CD127(low)CD4(+) T lymphocytes the CD25(high) T regulatory subset from conventional CD25(-) T lymphocytes. Functional studies reveal that addition of AY19 mAb enhances the proliferation of peripheral blood mononuclear cells (PBMC) obtained with mitogenic concentrations of PMA. Interestingly, we found that although AY19 mAb does not increase the optimal PBMC proliferation induced by a mitogenic pair of anti-CD2 mAbs it prolongs their time of proliferation. Thus, these results indicate that the anti-IL-1RAcP mAb AY19 exhibits unique functional properties by triggering co-stimulatory signals in lymphocytes.