P38 MAPK activation by Toxoplasma gondii secretory effector protein GRA24: Role in cytokine regulation and induction of protective immunity

Abstract

Abstract The apicomplexan Toxoplasma gondii induces strong protective immunity in part dependent upon recognition by Toll-like receptors (TLR) 11 and 12 operating in conjunction with MyD88 in the murine host. However, these TLR are not present in humans, prompting us to investigate MyD88-independent signaling pathways of resistance. We show here that parasite dense granule protein GRA24 induces p38 MAPK activation and subsequent IL-12 production in bone marrow derived mouse macrophages (BMDM). Furthermore, Toxoplasma triggers two waves of p38 MAPK phosphorylation in MyD88+/+ and MyD88−/− BMDM. Using the uracil auxotrophic Type I T. gondii strain cps1-1, we demonstrate that the late response does not require active parasite proliferation, but strictly depends upon GRA24. By i. p. inoculation with cps1-1 and cps1-1:Δgra24, we identified unique subsets of chemokines and cytokines that were up and down regulated by GRA24. We demonstrate that cps1-1 triggers a strong host-protective GRA24-dependent Th1 response in the absence of MyD88. To gain deeper insight into genes regulated by GRA24, we employed qPCR to evaluate expression of transcripts for 84 cytokine and chemokine genes in BMDM infected with GRA24-positive compared to GRA24-negative parasites. Our results identify a specific signature of genes regulated by this dense granule protein. In conjunction, using qPCR we evaluated the impact of GRA24 on a panel of macrophage long noncoding RNA (lncRNA) regulated by Toxoplasma infection. Taken together, our data identify GRA24 as a major mediator of p38 MAPK activation, induction of IL-12 and other immune mediators, as well as protective immunity that operates independently of the TLR/MyD88 cascade.