Neonatal CD19⁺B220^lo cells sense microbiota via TLR2/4 activation driving proliferation and differentiation

Neonatal respiratory infections remain a leading cause of mortality and morbidity worldwide, yet there is still little information about the role of B cell populations in the respiratory tract. CD19⁺B220^lo are B cells that are found in neonatal lungs, and antibiotic treatment of pregnant females and their offspring reduced the number of these cells and modified their VH immunoglobulin repertoire. Activation in vitro by the TLR2 (PAM3 and FSL1) and TLR4 (LPS) ligands induced proliferation and differentiation of neonatal B cells in the lung and spleen, generating CD138⁺ cells and releasing IgM, IgG1 and small amounts of IgA, as well as several cytokines. A higher frequency of CD138⁺ cells was observed within both purified CD19⁺B220^lo and CD19⁺B220^hi populations following in vitro TLR2/TLR1 stimulation with PAM3. The lung cultures adopted an inflammatory cytokine profile (TNF-α, IL1-α, IL6), as opposed to the regulatory profile (IL10) of spleen cultures. Taken together, these findings support the conclusion that neonatal lung and spleen CD19⁺B220^lo cells can sense the microbiota via both TLR2 and TLR4, thereby promoting their activation and differentiation.