NF kappa B Regulator Bcl3 Controls Development and Function of Classical Dendritic Cells inToxoplasma gondiiInfection

The atypical IκB family member Bcl3 associates with p50/NF-κB1 or p52/NF-κB2 homodimers in the nucleus, and positively or negatively modulates transcription in a context-dependent manner. In mice lacking Bcl3 globally or specifically in CD11c+cells,Toxoplasma gondiiinfection is uniformly fatal and is associated with an impaired Th1 immune response. Since Bcl3 expression in dendritic cells (DC) is pivotal for antigen presentation and since classical DCs (cDC) are major antigen presenting cells, we investigated the role of Bcl3 specifically in cDCs inT. gondiiinfectionin vivoby crossing Zbtb46 cre mice withBcl3flx/flxmice. The conditional cDC Bcl3 KO was as susceptible to lethalT. gondiiinfection as the total Bcl3 KO and generated poor Th1 responses. Splenocyte single cell RNA seq in the model revealed defective Bcl3-dependent expression of genes involved in antigen processing. Consistent with this, soluble toxoplasma antigen presentation was impaired in Bcl3-deficient cDCs, and tetramer staining demonstrated defectiveT. gondiiantigen-specific splenic CD4+and CD8+T cell responses in infected cDCBcl3-/-mice.In vitrodifferentiation of bone marrow progenitors from wildtype and cDCBcl3-/-mice using Flt3L, NOTCH and IFN-γ stimulation recapitulated the defective Bcl3-dependent cDC antigen-presentation activity observed in vivo. Splenocyte single cell RNA seq also revealed the existence of a unique subpopulation of Zbtb46+LysM+DC which exhibited Bcl3-dependent expansion after infection. We also detected cDCs coexpressing the monocytic markers CD64 and Ly6C (designated icDC1 and icDC2) mainly in infected spleen, which were less abundant inBcl3flx/flxZbtb46 cremice. Together, our results indicate that Bcl3 in classical DCs is a major determinant of protective T cell responses and survival inT. gondii-infected mice, and shapes DC ontogeny.Author SummaryDendritic cells initiate immune responses against invading pathogens. As professional antigen presenting cells they process and present antigen via the major histocompatibility complex to T cells and thus activate them. Bcl3, an atypical member of the IκB family regulates the APC function of dendritic cells. In this study we show that expression of Bcl3 specifically in classical DCs is critical for host protection against a protozoan parasite,Toxoplasma gondii. Host protective proinflammatory mechanisms are compromised in mice deficient in Bcl3 in classical DCs leading to an elevated organ parasite load and eventually death of the infected animals. We also found the emergence of Bcl3-dependent hybrid DCs uponT. gondiiinfection, which have mixed phenotypic markers from DCs and monocytes. Antigen processing genes are significantly downregulated in Bcl3-deficient cDCs, which may account for defective cross presentation ofT. gondiiantigens. In an in vitro differentiation model, we showed that development of XCR1+cross presenting cDC1s is critically regulated by Bcl3. Overall, this study reveals the complexity of dendritic cell ontogeny and the role of Bcl3 in classical DC function in the context ofToxoplasmainfection.