NK cells regulate CD4+ and CD8+ T cells in acute viral infection, vaccination, and the tumor microenvironment. NK cells also become exhausted in chronic activation settings. The mechanisms causing these ILC responses and their impact on adaptive immunity are unclear. CD8+ T cell exhaustion develops during chronic Toxoplasma gondii ( T. gondii ) infection resulting in parasite reactivation and death. How chronic T. gondii infection impacts the NK cell compartment is not known. We demonstrate that NK cells do not exhibit hallmarks of exhaustion. Their numbers are stable and they do not express high PD1 or LAG3. NK cell depletion with anti-NK1.1 is therapeutic and rescues chronic T. gondii infected mice from CD8+ T cell exhaustion dependent death, increases survival after lethal secondary challenge and alters cyst burdens in brain. Anti-NK1.1 treatment increased polyfunctional CD8+ T cell responses in spleen and brain and reduced CD8+ T cell apoptosis in spleen. Chronic T. gondii infection promotes the development of a modified NK cell compartment, which does not exhibit normal NK cell characteristics. NK cells are Ly49 and TRAIL negative and are enriched for expression of CD94/NKG2A and KLRG1. These NK cells are found in both spleen and brain. They do not produce IFNγ, are IL-10 negative, do not increase PDL1 expression, but do increase CD107a on their surface. Based on the NK cell receptor phenotype we observed NKp46 and CD94-NKG2A cognate ligands were measured. Activating NKp46 (NCR1-ligand) ligand increased and NKG2A ligand Qa-1b expression was reduced on CD8+ T cells. Blockade of NKp46 rescued the chronically infected mice from death and reduced the number of NKG2A+ cells. Immunization with a single dose non-persistent 100% protective T. gondii vaccination did not induce this cell population in the spleen, suggesting persistent infection is essential for their development. We hypothesize chronic T. gondii infection induces an NKp46 dependent modified NK cell population that reduces functional CD8+ T cells to promote persistent parasite infection in the brain. NK cell targeted therapies could enhance immunity in people with chronic infections, chronic inflammation and cancer.

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12NK cells regulate CD4+ and CD8+ T cells in acute viral infection, vaccination and the 13 tumor microenvironment. NK cells also become exhausted in chronic activation settings. 14 The mechanisms causing these ILC responses and their impact on adaptive immunity are 15 unclear. CD8+ T cell exhaustion develops during chronic Toxoplasma gondii (T. gondii) 16 infection resulting in parasite reactivation and death. How chronic T. gondii infection 17 impacts the NK cell compartment is not known. We demonstrate that NK cells do not 18 exhibit hallmarks of exhaustion. Their numbers are stable and they do not express high 19 PD1 or LAG3. NK cell depletion with anti-NK1.1 is therapeutic and rescues chronic T. 20 gondii infected mice from CD8+ T cell exhaustion dependent death, increases survival 21 after lethal secondary challenge and reduces parasite reactivation. Anti-NK1.1 treatment 22increased polyfunctional CD8+ T cell responses in spleen and brain and reduced CD8+ T 23 cell apoptosis. Chronic T. gondii infection promotes the development of a modified NK 24 cell compartment, which does not exhibit normal NK cell behavior. This splenic CD49a-25 CD49b+NKp46+ NK cell population develops during the early chronic phase of infection 26 and increases through the late chronic phase of infection. They are Ly49 and TRAIL 27 negative and are enriched for expression of CD94/NKG2A and KLRG1. They do not 28 produce IFNγ, are IL-10 negative, do not increase PDL1 expression, but do increase 29CD107a on their surface. They are also absent from brain. Based on the NK cell receptor 30 phenotype we observed NKp46 and CD94-NKG2A cognate ligands were measured. 31Activating NKp46 (NCR1-ligand) ligand increased and NKG2A ligand Qa-1b expression 32 was reduced. Blockade of NKp46 also rescued the chronically infected mice from death. 33Immunization with a single dose non-persistent 100% protective T. gondii vaccination 34 did not induce this cell population in the spleen, suggesting persistent infection is 35 essential for their development. We hypothesize chronic T. gondii infection induces an 36NKp46 dependent modified NK cell population that reduces functional CD8+ T cells to 37 promote persistent parasite infection in the brain. NK cell targeted therapies could 38 enhance immunity in people with chronic infections, chronic inflammation and cancer. 39 40 103 Materials and methods 104Mice 105 C57BL/6 (B6), B6.129S6-IL-10 tm1Flv /J (IL-10-GFP Tiger) mice were purchased from The 106 Jackson Laboratory. All animals were housed under specific pathogen-free conditions at 107 the University of Wyoming Animal Facility. This study was carried out in strict 108 accordance following the recommendations in the Guide for the Care and Use of 109 Laboratory Animals of the National Institutes of Health. The University of Wyoming 110 Institutional Animal Care and Use Committee (IACUC) (PHS/NIH/OLAW assurance 111 number: A3216-01) approved all animal protocols. 112 113 T. gondii parasites and infection 114 130 Brain and spleen T cell isolation and stimulatio...

Natural Killer (NK) cells can develop memory-like features and contribute to long-term immunity in mice and humans. NK cells are critical for protection against acute T. gondii infection. However, whether they contribute to long-term immunity in response to this parasite is unknown. We used a vaccine challenge model of parasite infection to address this question and to define the mechanism by which NK cells are activated during secondary parasite infection. We found NK cells were required for control of secondary infection. NK cells increased in number at the infection site, became cytotoxic and produced IFNγ. Adoptive transfer and NK-cell fate mapping revealed that T. gondii-experienced NK cells were not intrinsically different from naïve NK cells with respect to their long-term persistence and ability to protect. Thus, they did not develop memory-like characteristics. Instead, a cell-extrinsic mechanism may control protective NK-cell responses during secondary infection. To test the involvement of a cell-extrinsic mechanism, we used anti-IL-12p70 depletion and IL-12p35−/− mice and found that the secondary NK-cell response was not fully dependent on IL-12. IL-23 depletion with anti-IL-23p19 in vivo significantly reduced the secondary NK-cell response, suggesting that both IL-12 and IL-23 were involved. Anti-IL-12p40 treatment, which blocks both IL-12 and IL-23, eliminated the protective secondary NK-cell response, supporting this hypothesis. Our results define a previously unknown protective role for NK cells during secondary T. gondii infection that is dependent on both IL-12 and IL-23.