Nfkbid-driven B cell and antibody responses against T. gondii
Neglected parasitic diseases, which overlap with known diseases of poverty,cost over 30 million disability-adjusted life years (DALY) and are a massive economic burden for developing countries (Hotez et al., 2014). Toxoplasma gondii is the causative agent of toxoplasmosis, termed a "neglected parasitic diseases" and is a major source of foodborne mortality according to the Center for Disease Control and Prevention (CDC). T. gondii is a globally distributed parasite that infects 30% of the entire human population and no protective vaccine exists for this pathogen. T. gondii has a wide range of virulence; from non-virulent strains endemic to western Europe and North America to highly virulent and genetically diverse strains in South America which are known to evade immunological memory. The overarching goal of my thesis is to find host immunological mechanisms responsible for immunity to virulent T. gondii strains. First, attempts were made to reverse T cell exhaustion that we demonstrated occurs in chronically infected mice given a virulent secondary challenge, but a variety of checkpoint blockade therapies failed to provide immunity to challenge, suggesting alternative immunological mechanisms are required for immunity to secondary infection. We then used an unbiased forward genetic screen to test for survival against virulent secondary infections, and discovered 4 genomic loci associated with immunity. The immunity locus on chromosome 7 encompassed the gene Nfkbid which encodes the protein IkBNS, an atypical regulator of NF-kB signaling required for B-1 cell development. We found that mice without Nfkbid (bumble) could survive primary infection with a low virulent strain, but were susceptible to secondary infections with virulent strains. T. gondii-specific T cell responses for both CD4s and CD8s were intact in bumble mice. However, antibody responses to T. gondii were found to be deficient in bumble mice, specifically parasite-specific IgM was completely abrogated and all parasite-specific IgG isotypes were reduced. Using a series of adoptive transfer and mouse chimera strategies, we were able to make two major observations: (1) B-2 cells are responsible for the majority of high avidity T. gondii-specific antibody response and (2) B-1 cells are required for full immunity to T. gondii infection. In addition, humoral responses were found to be largely enhanced in resistant mice, with greater activation and class-switch recombination found in both the B-1 and B-2 B cell compartments. Finally, we present further evidence that the GPI-anchor of T. gondii may be a favored moiety by antibodies to recognize T. gondii parasite antigen. Together, our data suggests that, in addition to T cells, the anti-T. gondii immune response must be "layered" by both B-1 and B-2 B cells, to generate protective immunity and that Nfkbid is central to the immunological memory responses against T. gondii.