Neuropilin-1 Is Expressed on Highly Activated CD4(+) Effector T Cells and Dysfunctional CD4(+) Conventional T Cells from Naive Mice

Neuropilin-1 (Nrp-1) is a well described marker molecule for CD4(+)Foxp3(+) thymus-derived regulatory T cells (Tregs). In addition, a small population of CD4(+)Foxp3(-) conventional (conv) T cells expresses Nrp-1 in naive mice, and Nrp-1 expression has been described to be upregulated on activated CD4(+) T cells. However, the function of Nrp-1 expression on CD4(+) non-Tregs still remains elusive. In this study, we demonstrate that Nrp-1 expression was induced upon stimulation of CD4(+)Foxp3(-) T cells in vitro and during an ongoing immune response in vivo. This activation-induced Nrp-1(+)CD4(+) T cell subset (iNrp-1(+)) showed a highly activated phenotype in terms of elevated CD25 and CD44 expression, enhanced production of proinflammatory cytokines, and increased proliferation compared with the Nrp-1(-)CD4(+) counterpart. In contrast, Nrp-1(+)CD4(+)Foxp3(-) conv T cells from naive mice (nNrp-1(+)) were dysfunctional. nNrp-1(+)CD4(+) conv T cells upregulated activation-associated molecules to a lesser extent, exhibited impaired proliferation and produced fewer proinflammatory cytokines than Nrp-1(-)CD4(+) conv T cells upon stimulation in vitro. Moreover, the expression of PD-1 and CTLA-4 was significantly higher on nNrp-1(+)CD4(+)Foxp3(-) T cells compared with iNrp-1(+)CD4(+)Foxp3(-) T cells and Nrp-1(-)CD4(+)Foxp32 T cells after stimulation and under homeostatic conditions. Strikingly, transfer of Ag-specific iNrp-1(+)CD4(+) conv T cells aggravated diabetes development, whereas Ag-specific nNrp-1(+)CD4(+) conv T cells failed to induce disease in a T cell transfer model of diabetes. Overall, our results indicate that Nrp-1 expression has opposite functions in recently activated CD4(+) non-Tregs compared with CD4(+) non-Tregs from naive mice.