Distinct Differentiation Programs Triggered by IL-6 and LPS in Teleost IgM+ B Cells in The Absence of Germinal Centers

Abstract

Although originally identified as a B cell differentiation factor, it is now known that mammalian interleukin-6 (IL-6) only regulates B cells committed to plasma cells in response to T-dependent (TD) antigens within germinal centers (GCs). Even though adaptive immunity is present in teleost fish, these species lack lymph nodes and GCs. Thus, the aim of the present study was to establish the role of trout IL-6 on B cells, comparing its effects to those induced by bacterial lipopolysaccharide (LPS). We demonstrate that the effects of teleost IL-6 on naïve spleen B cells include proliferation, activation of NF-κB, increased IgM secretion, up-regulation of Blimp1 transcription and decreased MHC-II surface expression that point to trout IL-6 as a differentiation factor for IgM antibody-secreting cells (ASCs). However, LPS induced the secretion of IgM without up-regulating Blimp1, driving the cells towards an intermediate activation state in which antigen presenting mechanisms are elicited together with antibody secretion and expression of pro-inflammatory genes. Our results reveal that, in trout, IL-6 is a differentiation factor for B cells, stimulating IgM responses in the absence of follicular structures, and suggest that it was after follicular structures appeared that this cytokine evolved to modulate TD responses within the GC.

Characterization of BAFF and APRIL subfamily receptors in rainbow trout (Oncorhynchus mykiss). Potential role of the BAFF / APRIL axis in the pathogenesis of proliferative kidney disease

“…We included LPS in these experiments as a positive control, since LPS has been shown to induce proliferation and increase survival of trout IgM + B cells [45]. Surprisingly, only BALM was capable of significantly promoting the survival of IgM + B cells in the kidney after 3 days of incubation, at similar levels to those induced by LPS (Fig 9A).…”

What goes around comes around: an investigation of resistance to proliferative kidney disease in rainbow trout Oncorhynchus mykiss (Walbaum) following experimental re‐exposure

“…This study advocates that IL‐6 can act as a differentiation factor for IgM+ B cells towards ASCs (Abós et al . ). In view of this finding and our data showing a downregulation or transient expression of MHC II in the reinfection fish, we support the notion that the differentiation of teleost ASCs is not always dependent on MHC II expression.…”

Identification of the First Teleost CD5 Molecule: Additional Evidence on Phenotypical and Functional Similarities between Fish IgM+ B Cells and Mammalian B1 Cells

“…Prior to acquisition, DRAQ5 nuclear stain (Molecular Probes) was added as per manufacturer's recommendations. Data were collected on an Image-Stream MK II and analyzed using IDEAS software (Amnis), as previously described (56)(57)(58).…”

“…Because murine B1 cells fail to activate NF-kB upon BCR activation (64), we also analyzed the translocation of the NF-kB p65 subunit to the nucleus of splenocytes in response to BCR crosslinking in both murine and fish B cells. We performed this using a quantitative imaging flow cytometry-based approach previously used in rainbow trout (57,58). In mice, anti-IgM provoked a significant increase in the percentage of CD9 2 B cells, with p65 translocated to the nucleus after 30 min (Fig.…”