Porcine non-conventional B-1-like cells are a potent source of Streptococcus suis-binding IgM

Introduction: Streptococcus suis serotype (cps) 2 is an important bacterial pathogen in piglet nurseries and an emerging zoonotic agent without effective vaccines available. Immunoglobulin (Ig)M plays an essential role in host defense against S. suis. In mice, non-conventional B-1 cells are a major source of protective IgM against encapsulated bacterial pathogens, such as S. pneumoniae. Two IgM⁺CD21^- B-1-like cell subpopulations, distinguishable by CD11R1 expression, were described in pigs, but their properties and functions are poorly understood. This study aimed at a first characterization of the porcine early IgM B cell response against S. suis cps 2.

Methods: We analyzed the same healthy pigs, naturally colonized by different S. suis serotypes, including cps 2, at four and eight weeks of age serologically and determined the frequency of different peripheral B cell subpopulations by flow cytometry. Furthermore, we isolated conventional IgM⁺CD21⁺ B-2 cells as well as non-conventional B-1-like cell subpopulations from peripheral blood of eight-weeks-old pigs to evaluate their potential of IgM secretion in response to innate and adaptive stimuli in vitro.

Results: Between the fourth and eighth week of life, a characteristic increase of S. suis cps 2-binding serum IgM antibodies, restricting bacterial growth, was observed. Moreover, we show for the first time that the significant increase of anti-S. suis serum IgM is associated with a relative increase of peripheral non-conventional IgM⁺CD21^- B-1-like cells in vivo, particularly of the IgM⁺CD21^- CD11R1^- subpopulation. Noteworthy, sorted IgM⁺CD21^- CD11R1^- B-1-like cells from eight-weeks-old pigs spontaneously secreted IgM in vitro. In addition, both non-conventional IgM⁺CD21^- B cell subpopulations, in contrast to conventional IgM⁺CD21⁺ B-2 cells, produced anti-S. suis IgM upon toll-like receptor (TLR) stimulation underlining their innate-like characteristics. We furthermore observed that both B-1-like subpopulations secrete S. suis cps 2-binding IgM upon stimulation with T cell-associated factors with highest amounts in IgM⁺CD21^-CD11R1^- B-1-like cells even exceeding anti-S. suis IgM levels produced by B-2 cells.

Conclusion: Porcine non-conventional B-1-like cells are a potent source of S. suis-binding IgM indicating a role in immunity during a critical phase of piglet rearing.