Background/Purpose:  In rheumatoid arthritis (RA), synovial fibroblasts (SF) are one main contributor of joint destruction since they resist apoptosis and secrete pro-inflammatory cytokines and matrix degrading enzymes. Therefore, they are key players in directing immune responses in direct interaction with other cell types, like B cells. TNF superfamily members, like B cell activating factor of the TNF family (BAFF) are important B cell function modulators and survival factors and are produced by synovial fibroblasts under certain conditions. Since better understanding of B cell – fibroblast interaction will lead to potential new treatment targets, we started to characterize this interplay.

Methods:  Osteoarthritis (OA, n=5) and RA (n=5) synovial fibroblasts (SF) were co-cultured in passage 3-6 with B cells. Following a preincubation period of 48h with different inflammatory mediators (TNF, IL-1, IFN-γ) or B cell stimulating factors (BAFF, CpG), B cells were added to SF cultures and incubated for another 48h period. To determine B cell survival, we used AnnexinV/PI costain in FACS. AnnexinV/PI double negative B cells (living) are given as percentage of total B cells. To specifically block the action of BAFF, we used Belimumab.

Results:  The presence of SF increased B cell survival from 5.2% to 55.3% (p<0.001). RA SF as compared to OA SF showed similar capacity to increase B cell survival as compared to OA SF. However, conditioned media from RA SF showed increased capacity to enhance B cell survival as compared to OA SF supernatants (RA: 15.7% vs. OA: 11.1%, p=0.04). SF preincubation with INF-γ (survival 70.8%, p=0.015), IL-1 (survival 71.5%, p=0.018), or CpG (survival 87.7%, p=0.0002) further increased survival of B cells as compared to unstimulated SF. IFN-γ (0.1-50 ng/ml) induced BAFF from SF in a concentration dependent manner (ANOVA p<0.01). RA and OA SF showed the same response to INF-γ. Blocking BAFF by adding Belimumab (10µg/ml) decreased IFN induced survival to control values (survival 52.03%, p=0.58). Belimumab, INF-γ, or IL-1 in B cell only cultures had no effect on survival, respectively. After preincubation of SF with BAFF (0.1-10ng/ml), B cell survival was increased in a concentration dependent manner (ANOVA p<0.01, max. survival 78.7% with BAFF 10ng/ml) and inhibited in the presence of Belimumab (survival 64.9%, p=0.08).

Conclusion:  B cell survival is increased in the presence of RA an OA SF in a similar manner, although RA SF spontaneously produce more soluble B cell survival factors as compared to OA SF. Preincubating fibroblasts with BAFF, IFN-γ, IL-1, or CpG affects fibroblast function to further increase B cell survival. INF-γ- and BAFF-induced SF mediated increase in B cell survival can be blocked by Belimumab. Therefore, strategies to interfere with BAFF directly in the joint might lead to decreased B cell survival and reduced inflammation.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/synovial-fibroblasts-regulate-b-cell-survival-via-b-cell-activating-factor-of-the-tnf-family-baff/