Understanding the Heterogeneous Population of Age-Associated B Cells and Their Contributions to Autoimmunity and Immune Response to Pathogens

Impact of Fli1 deletion on B cell populations: A focus on age-associated B cells and transcriptional dynamics

BACKGROUND

Altered Fli1 expression is associated with various autoimmune diseases, yet its impact on B cells remains unexplored.

OBJECTIVE

This study investigated the direct effects of Fli1 depletion on B cell populations, focusing on age-associated B cells (ABCs).

METHODS

Splenocytes of Fli1 BcKO (Cd19-Cre+/-; Fli1flox/flox) and Cd19-Cre+/- mice were analyzed flow cytometrically. Transcriptional/epigenetic profiles of Cd11b+Cd11c+ ABCs were examined by RNA-sequencing and ATAC-sequencing.

RESULTS

Fli1 BcKO mice displayed a notable reduction in follicular and marginal zone B cells, with a concurrent rise in newly formed B cells compared to Cd19-Cre+/- mice. Additionally, a striking increase in B-1 B cells, as well as Cd11b+Cd11c+ or T-bet+Cd11c+ ABCs, was observed in Fli1 BcKO mice. Furthermore, these mice exhibited elevated Cd138 levels in follicular B cells. Conducting transcriptional analyses of Fli1-depleted ABCs unveiled upregulated genes associated with cell-cell adhesion, coupled with downregulated genes linked to cell activation or immune responses. Exploring the chromatin landscape found that Fli1 depletion dysregulated the chromatin accessibility of the interferon regulatory factor family, implying potential roles in autoimmunity.

CONCLUSION

These findings suggest complex modulations of B cell populations and immune-related gene expression due to Fli1 deficiency, shedding light on its involvement in autoimmune processes.

Increased development of T-bet+CD11c+ B cells predisposes to lupus in females: Analysis in BXD2 mouse and genetic crosses

Cardinal features of lupus include elevated B cell activation and autoantibody production with a female sex preponderance. We quantified interactions of sex and genetic variation on the development of autoimmune B-cell phenotypes and autoantibodies in the BXD2 murine model of lupus using a cohort of backcrossed progeny (BXD2 x C57BL/6J) x BXD2. Sex was the key factor leading to increased total IgG, IgG2b, and autoantibodies. The percentage of T-bet+CD11c+ IgD+ activated naive B cells (aNAV) was higher in females and was associated with increased T-bet+CD11c+ IgD- age-related B cells, Fas+GL7+ germinal center B cells, Cxcr5-Icos+ peripheral T-helper cells, and Cxcr5+Icos+ follicular T-helper cells. IFN-β was elevated in females. Variation in aNAV cells was mapped to Chr 7 in a locus that shows significant interactions between the female sex and heterozygous B/D variant. Our results suggest that activation of naive B cells forms the basis for the female-predominant development of autoantibodies in lupus-susceptible BXD2 mice.

IgD+ age-associated B cells are the progenitors of the main T-independent B cell response to infection that generates protective Ab and can be induced by an inactivated vaccine in the aged

Age-associated B cells (ABC) accumulate with age and are associated with autoimmunity and chronic infection. However, their contributions to acute infection in the aged and their developmental pathways are unclear. We find that the response against influenza A virus infection in aged mice is dominated by a Fas+ GL7- effector B cell population we call infection-induced ABC (iABC). Most iABC express IgM and include antibody-secreting cells in the spleen, lung, and bone marrow. We find that in response to influenza, IgD+ CD21- CD23- ABC are the precursors of iABC and become memory B cells. These IgD+ ABC develop in germ-free mice, so are independent of foreign antigen recognition. The response of ABC to influenza infection, resulting in iABC, is T cell independent and requires both extrinsic TLR7 and TLR9 signals. In response to influenza infection, IgD+ ABC can induce a faster recovery of weight and higher total anti-influenza IgG and IgM titers that can neutralize virus. Immunization with whole inactivated virus also generates iABC in aged mice. Thus, in unimmunized aged mice, whose other B and T cell responses have waned, IgD+ ABC are likely the naive B cells with the potential to become Ab-secreting cells and to provide protection from infection in the aged.

Age-Associated B Cell Features of the Murine High-Grade B Cell Lymphoma Bc.DLFL1 and Its Extranodal Expansion in Abdominal Adipose Tissues

Diffuse large B cell lymphoma comprises a heterogeneous group of B cell-derived tumors, with different degrees of aggressiveness, as defined by their cellular origin and tissue microenvironment. Using the spontaneous Bc.DLFL1 lymphoma originating from a BALB/c mouse as a diffuse large B cell lymphoma model, in this study we demonstrate that the lymphoma cells display surface phenotype, IgH V-region somatic mutations, transcription factor characteristics and in vivo location to splenic extrafollicular regions of age-associated B cells (ABCs), corresponding to T-bet⁺ and Blimp-1⁺/CD138^- plasmablasts derivation. The expansion of lymphoma cells within lymphoid tissues took place in a close arrangement with CD11c⁺ dendritic cells, whereas the extranodal infiltration occurred selectively in the mesentery and omentum containing resident gp38/podoplanin⁺ fibroblastic reticular cells. Antagonizing BAFF-R activity by mBR3-Fc soluble receptor fusion protein led to a significant delay of disease progression. The extranodal expansion of Bc.DLFL1 lymphoma within the omental and mesenteric adipose tissues was coupled with a significant change of the tissue cytokine landscape, including both shared alterations and tissue-specific variations. Our findings indicate that while Bc.DLFL1 cells of ABC origin retain the positioning pattern within lymphoid tissues of their physiological counterpart, they also expand in non-lymphoid tissues in a BAFF-dependent manner, where they may alter the adipose tissue microenvironment to support their extranodal growth.

Age-related immune alterations and cerebrovascular inflammation

Aging is associated with chronic systemic inflammation, which contributes to the development of many age-related diseases, including vascular disease. The world's population is aging, leading to an increasing prevalence of both stroke and vascular dementia. The inflammatory response to ischemic stroke is critical to both stroke pathophysiology and recovery. Age is a predictor of poor outcomes after stroke. The immune response to stroke is altered in aged individuals, which contributes to the disparate outcomes between young and aged patients. In this review, we describe the current knowledge of the effects of aging on the immune system and the cerebral vasculature and how these changes alter the immune response to stroke and vascular dementia in animal and human studies. Potential implications of these age-related immune alterations on chronic inflammation in vascular disease outcome are highlighted.

"An Intrinsic Program Determines Key Age-Associated Changes in Adaptive Immunity that Limit Response to Non-Pathogens."

As mice age their adaptive immune system changes dramatically, leading to weakened responses to newly encountered antigens and poor efficacy of vaccines. A shared pattern emerges in the aged, with both CD4 T and B cell responses requiring higher levels of pathogen recognition. Moreover, in aged germ-free mice we find accumulation of the same novel age-associated T and B cell subsets that we and others have previously identified using mice maintained in normal laboratory animal housing conditions, suggesting that their development follows an intrinsic program.