We found that T. cruzi infection led to increased expression of PD-1 and its ligands on peritoneal Mφs as well as during in vitro infection. On the other hand, F4/80+ Mφs from T. cruzi-infected mice suppressed the proliferative response of naive CD90.2+ T cells to primary stimulation with Con A. The PD-L1 or PD-1 blockade significantly reduced the suppressive
activity of T. cruzi-infected-Mφs, indicating that PD-L1 is directly involved in their suppressive activity. However, PD-L2 blockade was not able to restore the T-cell proliferation suppressed by T. cruzi-infected Mφs. Given that it has been demonstrated that PD-L2 Carfilzomib nmr KO mice show an increase in Th2 response,47,48 we decided to evaluate if PD-L2 blockade was able to induce Arg I. Involvement of Arg I in the suppressive lambrolizumab capacity of Mφs has been broadly demonstrated.26,27,60 Our data showed that PD-L2 blockade in T. cruzi-infected Mφs induced Arg I activity and expression that might explain the immunosuppressive capacity of these Mφs. However, we did not see changes in Arg I expression and activity in cell cultures treated with PD-1 or with PD-L1 blocking antibodies. Therefore,
in our T. cruzi infection model the immunosuppression may be directly mediated by PD-1/PD-L1 and indirectly by PD-L2 through Arg I regulation. Moreover, supporting data demonstrated that Arg I plays a key role in T-cell suppression in non-healing leishmaniasis lesions.26 Arg I, through the local depletion of l-arginine, impairs at the site of lesions the capacity of T cells to proliferate and to produce IFN-γ, which is required for parasite elimination.26 However, during Schistosoma mansoni infection, Arg I from Mφs favours the recovery of the infection by inhibiting T CD4+ cells and the production of cytokines.
The authors demonstrated Org 27569 that the primary suppressor mechanism was the depletion of arginine by Arg I from Mφs.60 Here, we show that PD-L2 blockade as well as PD-L2 deficiency enhances Arg, leading to an increase in parasite proliferation. In addition, Terrazas et al.38 have shown that Taenia crassiceps-induced Mφs were able to suppress T-cell proliferation through PD-L1 and PD-L2 up-regulation on Mφs in an IL-10, IFN-γ, NO independent and cell-to-cell contact dependent manner. In addition, Schistosoma mansoni worms selectively up-regulate PD-L1 to reduce T-cell activation during early acute stages of infection before the subsequent emergence of egg-induced T-cell suppression in the chronic stages of infection.61 Recently, It was shown that IL-4-stimulated Mφs up-regulate PD-L2 and the T-cell suppression induced by these Mφs was restored by adding anti-PD-L2 blocking antibodies.62 Therefore, T-cell suppression could be mediated by PD-L1 or PD-L2, depending on the manner in which Mφs are stimulated.