Background/Purpose: Recently, a population of CD11c+ age-associated B cells (ABCs) was identified in normal aged female mice. These cells could be expanded following activation by TLR7 agonists and their presence was associated with autoantibody production. The goals of the current study were to determine: a) when ABCs develop during B cell maturation in the spleen; b) how B cell intrinsic and extrinsic factors influence ABC generation; and c) how TLR7 expression influences ABC generation in a B cell receptor (BCR) transgenic (Tg) mouse with specificity for RNA (the TLR7 ligand).

Methods: Knock-in mice that overexpressed TLR7 on a B6 background (TLR7 Tg) were crossed with either IFNaR knockout (KO), RNAse Tg, HEL-specific BCR Tg, or RNA specific BCR Tg (H564) mice. Mixed chimeras were generated with combinations of wildtype B6 and TLR7 Tg bone marrow. Flow cytometry was used to identify splenic B cell subsets and analyze CD11c expression. CD11c+ (ABCs) and CD11c- (non-ABCs) B cells were sorted using a BD FACS Aria, and cultured for 6d with or without TLRs 4, 7, or 9 agonists. Ig production was quantified by ELISA.

Results: While in wildtype mice, CD11c+ ABCs were increased only in females greater than 6 months of age, TLR7 Tg mice had significantly expanded ABCs as young as 3 months in both males and females (1.1 ± 0.2 % in B6 versus 3.4 ± 1.1 % in TLR7 Tg, p < 0.003). ABC accumulation was greatest in the transitional B cell stages but the relative proportion of ABCs was also higher in marginal zone than follicular B cells. To address whether generation of ABCs was intrinsic or extrinsic we used WT:TLR7 Tg mixed bone marrow chimeras, and observed that ABC expansion of TLR7 Tg B cells had a definite cell-intrinsic component. However, when TLR7 Tg mice were crossed with RNAse Tg or IFNaR KO mice to remove the RNA ligand or response to IFN-I respectively, ABCs were modestly but statistically significantly reduced. In TLR7 Tg mice, serum levels of anti-RNA autoantibody correlated with % ABCs in spleen and we also observed that ABCs produced more antibody (including anti-RNA autoantibodies) in response to innate stimulation in vitro. Consistent with these findings, crossing TLR7 Tg and autoreactive H564 BCR mice further increased ABC accumulation, while crossing to an HEL mouse resulted in expansion of ABCs in the polyclonal but not in the HEL-specific subset.

Conclusion: CD11c+ ABCs are B cells that appear early during B cell maturation, and are increased under conditions of TLR7 hyperreactivity. ABC generation is, in part, cell intrinsic but is also influenced by exposure to self antigen and to IFN-I. ABCs can be directly implicated in autoantibody production by comparing antibody concentrations following stimulation of ABCs and non-ABCs in vitro as well as by enhanced ABC expansion in a TLR7 Tg mice that has specificity to self antigen (RNA) but not to non-self (HEL). Since these results suggested that ABCs, at least in part, contribute to autoantibody production, CD11c+ B cells may be a good target for amelioration of autoimmunity without broadly impairing host defense.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/differentiation-activation-and-autoreactivity-of-cd11c-b-cells-abcs/