Importantly, the specificity of such Treg has not been addressed. Influenza A virus infections have caused many Crenolanib supplier pandemics 11. Infections with this virus are acute and characterized by acute onset of fever, myalgias

and respiratory symptoms 12. Data in experimental mouse models showed that immune control of influenza infection is associated with the production of IFN-γ at the start and then followed by a peak in IL-10 when viral infection becomes controlled 13. IL-10 is well known for its anti-inflammatory effects and is known to limit and ultimately terminate inflammatory responses 14. In the mouse model, influenza-specific immunity comprises not only influenza-specific CD4+ Th1 cells, but also a subset of influenza-specific CD4+ T cells able to produce IFN-γ and IL-10, simultaneously 15. Interestingly, this cytokine profile resembles that of previously described adaptive Treg found in chronic diseases 5, 7, suggesting that such influenza-specific CD4+ T cells may in fact comprise Treg. In order to study if the immune

response to viruses causing acute infections also comprised virus-specific Treg, we set out to study the influenza-specific CD4+ T-cell response in healthy individuals. We show that in these individuals T-cell immunity to influenza is characterized by the production of both IFN-γ Protein Tyrosine Kinase inhibitor and IL-10. Isolated IL-10 and IFN-γ-producing T-cell clones displayed an immunosuppressive signature, as they were able to suppress CD4+ and CD8+ T cells when stimulated with influenza virus by interfering with the IL-2 pathway. These data show that virus-specific Treg can also be induced by viruses that are cleared by the immune system. The immune response to influenza infection in mice is characterized by a first wave of IFN-γ and is followed by IL-10 when the viral infection is controlled 13. This immune response not necessarily reflects the contraction of populations of T cells (e.g. Th1 and Th2) as one single influenza-specific

CD4+ T cell can produce both IFN-γ and IL-10 Sinomenine in mice 15. To study whether similar responses could also be observed in humans, the influenza-specific T-cell response in healthy individuals was analyzed. We focused on the natural response to influenza matrix 1 (M1) protein, as we had previously observed that M1-specific T cells could be detected directly ex vivo in the majority of individuals 16–19. Moreover, M1 is not included in influenza vaccines, thus allowing us to analyze the spontaneous response to influenza. Freshly isolated PBMC from healthy blood bank donors were stimulated with a pool of influenza M1 peptides. M1-specific responses were detected against multiple peptides, indicating that a broad T-cell response was mounted against influenza in these donors (Fig. 1A).