Blockade of LAG-3 in PD-L1-deficient mice enhances clearance of blood stage malaria independent of humoral responses

T cells expressing high levels of inhibitory receptors such as PD-1 and LAG-3 are a hallmark of chronic infections and cancer. Checkpoint blockade therapies targeting these receptors have been largely validated as promising strategies to restore exhausted T cell functions and clearance of chronic infections and tumors. The inability to develop long-term natural immunity in malaria-infected patients has been proposed to be at least partially accounted for by sustained expression of high levels of inhibitory receptors on T and B lymphocytes. While blockade or lack of PD-1/PD-L1 and/or LAG-3 was reported to promote better clearance of Plasmodium parasites in mice, how exactly these pathways contributes to protection is not known. Herein, using a mouse model of non-lethal P. yoelii (Py) infection, we reveal that the kinetics of blood parasitemia is indistinguishable between PD-1-/-, PD-L1-/- and WT mice. Yet, monoclonal antibody (mAb) blockade of LAG-3 in PD-L1-/- mice promoted accelerated control of blood parasite growth and clearance. We also report that i) the majority of LAG-3+ cells are T cells, ii) selective depletion of CD8+ T cells did not prevent anti-LAG-3-mediated protection, and iii) production of effector cytokines by CD4+ T cells is increased in anti-LAG-3-treated versus control mice. In addition, parasite-specific Ab serum titers and their ability to transfer protection from both groups of mice was comparable and depletion of CD4+ T cells prevented protection. Thus, taken together, these results are consistent with a model in which disruption of PD-L1 and LAG-3 on parasite-specific CD4+ T cells unleashes their ability to effectively clear blood parasites, independently from humoral responses. Author SummaryMalaria, caused by Plasmodium parasites, is a global burden for which an efficacious vaccine is urgently needed. The development of long-term immunity against malaria is unclear, but we know that both T and B (that produce antibodies, Ab) lymphocytes, that are subsets of white blood cells, are required. Studies in mouse models of malaria have suggested that sets of inhibitory receptors, namely LAG-3 and PD-1, expressed on cytotoxic and helper T lymphocytes hamper the development of effective immunity against malaria. Therapeutic blockade of these receptors was reported to enhance blood parasite clearance through the development of more protective parasite-specific helper T lymphocytes and Abs. Herein, we reveal that, while mice genetically deficient for the PD-1 pathway fail to clear blood parasites better than WT counterparts, anti-LAG-3 treatment does. Importantly, we found comparable parasite-specific Ab responses between all mouse groups, and Ab transfers conferred similar protection to newly infected mice. We also show that LAG-3 is mostly expressed on T lymphocytes, and that cytotoxic T lymphocytes are not involved in anti-LAG-3 accelerated clearance of parasites. Our results suggest that LAG-3 blockade acts on helper T lymphocytes to unleash their effector responses and enhance the control of blood-stage malaria, independently from parasite-specific Abs.