Absence of B Cell Function Impairs Protective Mucosal Immunity to Brucellosis

Abstract Mucosal vaccination with a live attenuated B. melitensis double-mutant (BMDM) confers sterilizing protection against pulmonary challenge with wild-type (wt) BM 16M. Although T cell-dependent immunity is essential for protection, we queried the role of B cells in this response. We hypothesized that B cells enable BMDM to persist in the lungs for stimulation of robust adaptive responses. Using BALB/c B cell-deficient (μMT) mice, early and long-term immunity subsequent mucosal vaccination was examined. Groups of BALB/c and μMT mice were orally primed, nasally boosted, and 4 wks later, splenic and lung lymphocytes were antigen-pulsed to determine cytokine responses. μMT mice showed significantly reduced pulmonary proinflammatory responses by three-fold relative to BALB/c mice. Nasal challenge with wt BM 16M of BMDM-vaccinated mice resulted in impaired protective immunity in μMT mice evidenced by the significantly increased brucellae colonization of lungs and spleens. To determine the types of myeloid cells involved in brucellae recognition, μMT mice nasally vaccinated with a mCherry variant of BMDM showed a 23-fold reduction in lung mCherry expression relative to similarly vaccinated BALB/c mice at 5 days post-immunization. mCherry expression associated with various myeloid and B cells. Interestingly, although antigen-presenting cells (APCs) increased similarly in both groups of mice, mCherry expression in lung neutrophils and monocyte-derived macrophages in BALB/c mice was 23- and 4-fold enhanced, respectively, to μMT mice. We believe B cell retention of BMDM mutant enhances APC activity within the lungs facilitating the development of potent local T cell responses. Work supported by NIH R01 AI123244 and AI124244-03S1.