Antigen-specific activation thresholds of CD8⁺ T cells are independent of IFN-I-mediated partial lymphocyte activation

Type-I IFN (IFN-I) are highly pleiotropic cytokines known to modulate immune responses and play an early central role in mediating antiviral defenses. We have shown that IFN-I mediate transient up-regulation of a distinct subset of lymphocyte surface activation markers on both B and T cells in vivo independent of cognate antigen: a state referred to as 'partial lymphocyte activation'. Here we investigated in vitro the possibility that partial lymphocyte activation may serve to lower the antigen-specific activation thresholds for T cells. We found that the kinetics of Ca²⁺ flux in T cells responding to TCR cross-linking was not enhanced in partially activated T cells. Furthermore, following TCR stimulation with anti-cluster of differentiation (CD) 3ε, a lower proportion of partially activated than naive T cells proliferated. In contrast, the proliferation of partially activated and naive ovalbumin peptide (OVAp, SIINFEKL) specific CD8⁺ T cells (OT-I CD8⁺ T cells) was similar when stimulated with OVAp. Surprisingly, using an enzyme-linked immunospot (ELISPOT) assay for IFN-γ secretion, we found that a higher number of partially activated OT-I CD8⁺ T cells expressed effector functions than did naive OT-I CD8⁺ T cells. This is most readily explained by an increased survival of activated antigen-specific CD8⁺ T cells from a pool of partially activated T cells than naive T cells. Overall, when examining the effects of early (Ca²⁺ flux), intermediate (proliferation) or late events (IFN-γ secretion) of T-cell activation, we found that partial activation promotes the survival but does not alter the antigen-specific activation thresholds of CD8⁺ T cells.