Summary: Lymphocyte activation gene-3 (LAG-3) is an inhibitory receptor expressed by CD4+ T cells and tempers their homeostatic expansion. Because CD4+ T cell proliferation is tightly coupled to bioenergetics, we investigate the role of LAG-3 in modulating naive CD4+ T cell metabolism. LAG-3 deficiency enhances the metabolic profile of naive CD4+ T cells by elevating levels of mitochondrial biogenesis. In vivo, LAG-3 blockade partially restores expansion and the metabolic phenotype of wild-type CD4+ T cells to levels of Lag3−/− CD4+ T cells, solidifying that LAG-3 controls these processes. Lag3−/− CD4+ T cells also demonstrate greater signal transducer and activator of transcription 5 (STAT5) activation, enabling resistance to interleukin-7 (IL-7) deprivation. These results implicate this pathway as a target of LAG-3-mediated inhibition. Additionally, enhancement of STAT5 activation, as a result of LAG-3 deficiency, contributes to greater activation potential in these cells. These results identify an additional mode of regulation elicited by LAG-3 in controlling CD4+ T cell responses. : Previte et al. show that LAG-3 expression regulates the metabolic profile of naive CD4+ T cells during homeostatic expansion. They observed that Lag3-deficient CD4+ T cells are resistant to Interleukin-7 deprivation due to enhanced STAT5 activation. Increased STAT5 signaling also mediated greater activation potential in these T cells following stimulation. Keywords: LAG-3, CD4+ T cell, metabolism, mitochondria, STAT5
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