Mpeg1 exerts direct bactericidal activity and mediates innate antibacterial function of B cells in Nile tilapia

Mpeg1, a gene predominantly expressed in macrophages, belongs to the membrane attack complex/perforin (MACPF) superfamily. It serves as a critical pore-forming effector protein, playing a central role in the innate immune system's defense mechanisms against pathogenic invasion. Beyond serving as a macrophage-specific marker, Mpeg1 is also expressed in teleost IgM + B cells, with particularly high levels observed in head kidney (HK) granulocyte-monocyte/macrophage (G-M) gate IgM + plasma-like cells. Although Mpeg1 expression has been identified in teleost IgM + B cells, its specific immunoregulatory role within these cells remains elusive. Here, using the Nile tilapia ( Oreochromis niloticus ) model, we have demonstrated that On Mpeg1 contributes to innate immunity and modulates IgM + B cells, revealing a novel aspect of its function in antimicrobial defense. Our results showed that pathogenic bacterial challenge significantly upregulated On Mpeg1 expression in immune tissues, macrophages, and IgM + B cells. Recombinant On Mpeg1 ((r) On Mpeg1) and its isolated MACPF domain protein demonstrated direct bactericidal activity, with the full-length protein showing superior efficacy. In vivo targeted knockdown of OnMpeg1 increased bacterial loads, dysregulated inflammatory responses and reduced host survival. These effects were rescued by supplementation with (r) On Mpeg1. Crucially, On Mpeg1 knockdown in HK IgM + B cells severely impaired their intrinsic bactericidal capacity in both lymphoid (L) gate and G-M gate. We concluded that On Mpeg1 is a critical immune effector that not only directly kills pathogens but also potentiates the antimicrobial function of IgM + B cells, revealing a novel role for Mpeg1 in mediating innate bactericidal effector functions of B cells in teleost fish.