Naïve helper T cells with high CD5 expression have increased calcium signaling.

The adaptive immune response is orchestrated by T helper cells and their function is dependent upon interactions between the T cell receptor (TCR), peptide MHC (pMHC) and co-receptors. TCR-pMHC interactions initiate calcium signaling cascades which determine T cell activation, survival, proliferation and differentiation. CD5 is a co-receptor that plays an important role in regulating T cell signaling and fate during thymocyte education. CD5 surface expression on mature single positive thymocytes correlates with the TCR signal strength for positive selecting self-ligands. CD5 also plays a role in T cell function after thymic development is complete. Peripheral T cells with higher CD5 expression respond better to foreign antigen than those with lower CD5 expression and CD5-high T cells are enriched in memory populations. In our study, we examined the role of CD5 expression and calcium signaling in the primary response of T cells using two Listeria monocytogenes specific T helper cells (LLO118 and LLO56). These T cells recognize the same immunodominant epitope (LLO190-205) of L. monocytogenes and have divergent primary and secondary responses and different levels of CD5 expression. We found that each T cell has unique calcium mobilization in response to in vitro stimulation with LLO190-205 and that CD5 expression levels in these cells changed over time following stimulation. LLO56 naïve T helper cells, which expresses higher levels of CD5, have higher calcium mobilization than naïve LLO118 T cells. Three days after in vitro stimulation, LLO118 T cells had more robust calcium mobilization than LLO56 and there were no differences in calcium mobilization 8 days after in vitro stimulation. To further evaluate the role of CD5, we measured calcium signaling in CD5 knockout LLO118 and LLO56 T cells at these three time points and found that CD5 plays a significant role in promoting the calcium signaling of naïve CD5-high LLO56 T cells.