Background/Purpose: Marginal zone macrophages (MZMs), a small subset of specialized splenic macrophages located in the MZ, act as final follicular entry barrier to clear apoptotic cells (ACs) to prevent AC antigens (Ags) entering to the spleen follicles.  We recently reported reduced tolerogenic function, frequency and numbers of MZMs in the spleen of lupus prone BXD2 mice. Loss of MZMs were further confirmed in B6 Sle1.Sle2.Sle3 mice and lupus patients. Expression of lymphotoxin (LT) by B cells has been implicated as a critical factor to maintain MZMs. The present study investigated how type I interferons (IFNs) dissociated this interaction and induced MZM loss in lupus.

Methods: Confocal microscope and FACS analysis were carried out to determine the percentages and location of of MZMs, B cells and the distribution of LT and its receptor (LTβR) in the spleen. A mixed bone marrow (BXD2-GFP⁺Ifnαr^+/+:BXD2-GFP^–Ifnαr^-/- =1:1) reconstitution in BXD2 Rag2^-/-  mice was carried out to determine if type I IFNs affect MZ integrity through direct action on MZMs or indirectly by affecting B cells.  LT-LTβR interaction in vivo was disrupted by repeated injection of mice with a low dose of LTR fusion protein (LTR:Fc, 15 ug/mouse per week, 4 weeks) or by administrations of CpG (2.5 ug/mouse, every other day for 2 weeks).

Results: Repeated injections of CpG to B6, but not B6-Ifnαr^-/- mice, induced loss of MZMs. Consistent with this, the spontaneous loss of MZMs and the elevated autoantibodies against MZM Ags, Marco and scavenger receptor A, in BXD2 mice were prevented in the absence of type I IFN signaling.  Marginal zone B cells (MZs) are the major LT expressing cells in the marginal zone. Although there was comparable expression of LT, MZs were mainly localized in the follicles in BXD2 mice but in the marginal zone in both B6 and BXD2-Ifnαr^-/- mice. BM reconstitution experiment further shows that the absence of type I IFNR did not directly affect MZM repopulation, but rather affected MZ B cell distribution. GFP⁺Ifnαr^+/+ B cells mainly migrated to the follicles whereas GFP^–Ifnαr^-/-B cells remained in the marginal zone and became the source of LT to support MZMs. Administration of a low dose of LTR:Fc to BXD2 mice selectively depleted MZMs and enhanced the spontaneous germinal center (GC) response without affecting follicular dendritic cell networks. 

Conclusion: Our present study suggests a novel mechanism associated with type I IFN-promoted immune response against apoptotic self-Ags as a result of follicular migration of LT expressing and self-antigen carrier B cells. The shift of LT expressing B cells from the MZ to the follicles by type I IFNs promoted apoptotic Ag delivery and LT stimulation to FDCs via breaking down the marginal zone barrier and finally exaggerated the production of pathogenic auto-antibodies.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/follicular-entry-of-lymphotoxin-expressing-b-cells-via-type-i-interferon-disrupts-marginal-zone-barrier-integrity-and-exacerbates-systemic-autoimmunity/