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.
Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
(Copyright © 2024 Seidel, Kauffold, Protschka, Baums, Alber and Eschke.)