MyD88- and Bruton's tyrosine kinase-mediated signals are essential for T cell-independent pathogen-specific IgM...

Bacteremia is one of the leading causes of death by infectious disease. To understand the immune mechanisms required for the rapid control of bacteremia, we studied Borrelia hermsii, a bacterial pathogen that colonizes the blood stream of humans and rodents to an extremely high density. A T cell-independent IgM response is essential and sufficient for controlling B. hermsii bacteremia. Mice deficient in Bruton's tyrosine kinase (Btk), despite their known defect in BCR signaling, generated B. hermsii-specific IgM and resolved bacteremia, suggesting that an alternative activation or costimulatory pathway remained functional for T cell-independent B cells in Btk(-/-) mice. B. hermsii contains putative ligands for TLRs, and we found that mice deficient in TLR1, TLR2, or the TLR adaptor MyD88 generated anti-B. hermsii IgM with delayed kinetics and suffered more severe episodes of bacteremia. In striking contrast to the anti-B. hermsii IgM response in mice deficient only in Btk, mice deficient in both Btk and MyD88 were entirely incapable of generating B. hermsii-specific Ab or resolving bacteremia. The response to a T cell-dependent model Ag was unaffected in Btk(-/-) x MyD88(-/-) mice. These results suggest that MyD88 specifically promotes T cell-independent BCR signaling and that, in the absence of Btk, this TLR-mediated stimulation is a required component of this signal.