Lymphocyte Activation Gene 3 (LAG-3) Modulates the Ability of CD4 T-cells to Be Suppressed <i>In Vivo</i>

<div><p>Lymphocyte Activation Gene – 3 (LAG-3) is an immune checkpoint molecule that regulates both T-cell activation and homeostasis. However, the molecular mechanisms underlying LAG-3’s function are generally unknown. Using a model in which LAG-3 blockade or absence reliably augmented homeostatic proliferation <i>in vivo,</i> we found that IL-2 and STAT5 are critical for LAG-3 function. Similarly, LAG-3 blockade was ineffective in the absence of regulatory T-cells (Treg), suggesting an important role for LAG-3 in either the responsiveness of conventional T-cells (Tconv) to regulation, or a relative defect in the ability of LAG-3 KO regulatory T-cells (Treg) to suppress the proliferation of Tconv. In this model, LAG-3 KO Treg suppressed proliferation in a manner fairly similar to wild-type (WT) Treg, but LAG-3 KO Tconv were relatively resistant to suppression. Further studies also identified a role for LAG-3 in the induction/expansion of Treg. Finally, we found that LAG-3 blockade (or knockout) led to a relative skewing of naïve CD4 T-cells toward a T_H1 phenotype both <i>in vitro</i> and in <i>in vivo</i>. Together, these data suggest that LAG-3 expression on Tconv cells makes them more susceptible to Treg based suppression, and also regulates the development of a T_H1 T-cell response.</p>