Extrafollicular activation of B cells contributes to the long-term survival of CD28KO mice infected with Sylvio X10/4 Trypanosoma cruzi parasites

In the initial phase of infections, prior to the development of the follicular antibody response, an extrafollicular B cell response develops in lymphoid organs, with activation of cells displaying B cell receptors with low affinity to the different epitopes of the invading pathogen. To evaluate if this response allows early protection in the murine host, we investigated its role during infection with Sylvio X10/4 (a low virulence Trypanosoma cruzi parasite) of CD28KO mice, a mouse strain that does not develop a follicular antibody response, in parallel with infected C57BL/6 mice (used as control). We observed that T. cruzi infection allowed CD28KO mice to survive for a long period of time, ranging from a few weeks to almost a year. However, the CD28KO mice showed higher levels of subpatent parasitemia than C57BL/6 mice, as well as more intense and long-lasting inflammatory infiltrates in the heart. Following the CD28KO mice throughout the infection, we found a significant reduction in the number of CD8 ⁺ T cells in the spleen, but not CD4 ⁺ T cells, as well as a strong fluctuation in the number of B cells. Meanwhile, although the production of IFN-γ in the spleen was not affected by CD28 deficiency, a reduction in IL-2 production was observed in anti-CD3 stimulated CD4 ⁺ T cells from infected CD28KO mice. Regarding the humoral response, after confirming that the T follicular helper (TFH) cells and germinal center B cells (GC-B) were absent in the infected CD28KO mice, we demonstrated that the extrafollicular antibodies, present in CD28KO infected mice, allowed an early protection in the murine host, since the transfer of serum from chronic CD28KO mice to naïve CD28KO mice, which were infected with T. cruzi 24 hours later, promoted a significant reduction in parasitemia. These results demonstrate that low-affinity anti- T cruzi extrafollicular antibodies contribute to the control of circulating parasites, allowing Sylvio X10/4-infected CD28KO mice to survive for a long time.