Fig. 10: Model for a new function of omental neutrophil-secreted IL-1RA in limiting T cell responsiveness during the chronic stage of B. abortus infection.

Upon B. abortus infection, the bacteria replicate in omental macrophages and persist until the chronic stage in the omentum, identified as a pathogenic reservoir. B. abortus mobilizes macrophages, monocytes, and neutrophils to the peritoneal cavity and the omentum, nearby milky spots. These cells harbor an immunosuppressive phenotype, characterized by the co-expression of PD-L1 and Sca-1 that remains from the acute to the chronic phase of infection and is driven by B. abortus LPS. Maintenance of these immunosuppressive cells in the omentum depends on the wadC-encoded determinant of Brucella LPS. IL-1RA, which is notably secreted by PD-L1⁺Sca-1⁺ omental neutrophils and impairs CD4⁺ and CD8⁺ T cell responsiveness, is produced until the chronic phase of infection in the omentum, most likely favoring bacterial persistence. The illustration was created by Biorender.com.