Background/Purpose: Abnormal expression of the tyrosine kinase ZAP-70 by tumoral B cells in chronic lymphocytic leukemia (CLL) is associated with B cell receptor (BCR) hypersignalling and autoimmune cytopenia (AIC), mostly induced by polyclonal IgG from non-tumoral B cells. We previously shown that ZAP-70 is also expressed by residual non tumoral B cells in CLL and associated with AIC occurrence. We therefore hypothesized that ZAP-70 expression in non tumoral B cells could generally predispose to tolerance breakdown.

Methods: Ig heavy and light chain were amplified from FACS-sorted non tumoral mature switched ZAP-70⁺ B cells of CLL patients with AIC, for production of recombinant antibodies (rAb). Reactivity of these rAB was tested in ELISA and on HEp-2 cells. A knock in Zap-70⁺/Mb1-Cre⁺mouse model (KI ZAP) was generated to conditionally express ZAP-70 in B cells since proB stage, to study functional consequences of ZAP-70 expression in B cells in vivo. Zap-70⁺/Mb1-Cre^–mice were used as controls (CTRL).

Results: Among the 13 rAb produced to date from 5 different patients, 2 (15,5%) have an antinuclear autoreactivity and two others (15,5%) were polyreactive (DNA, lipopolysaccharide, insulin), compared respectively to 6% and 4,3% of control B cells. Compared to CTRL, microarrays revealed that KI ZAP mice were significantly enriched in circulating IgG and IgM autoantibodies against various antigens (mainly nuclear). These mice had reduced apoptosis rates of proB, preB and immatures B cells, enrichment in marginal zone (p=0.01) and reduction in B1a B cells (p=0.008). In the other part, KI ZAP mice had persistent hypogammaglobulinemia since 16 weeks-old (p=0.003 for IgG and p=0.03 for IgM), together with a reduction in matures naive, matures switched as well as in germinal center B cells (p=0.001, p=0.002 and p<0.001 respectively) and a trend was observed for plasma cells (p=0.07). After immunization by ovalbumin + Freund’s adjuvant, a reduced production of specific IgG and IgM was observed (p=0.01 and p=0.03 respectively). KI ZAP B cells shown increased spontaneous activation and proliferation levels holding after BCR stimulation, as well as an increased intracellular calcic flow. Preliminary data suggested a reduced SYK phosphorylation after BCR stimulation in KI ZAP B cells compared to CTRL.

Conclusion: We report for the first time that non tumoral ZAP-70⁺ B cells could be enriching in autoreactive cells in vitroand that ZAP-70 expression in B cells is associated in vivowith medullar selective advantage, enrichment in circulating autoreactive Ig and potential autoreactive B cells (marginal zone) with reduced immunoregulatory B cells (B1a). This expression is also associated with partial block in B cells peripheral development, but with a conversely early increased activation and proliferation status. ZAP-70 could interfere early with SYK leading to an altered BCR signaling responsible for defect in normal B maturation promoting emergence of autoreactive B cells. Mechanistic role of ZAP-70 in BCR signaling has to be further analyzed but our data open new opportunities involving ZAP-70 in the understanding of B cell development and physiopathology of tolerance breakdown.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/abnormal-zap-70-expression-in-b-cells-new-potential-role-in-tolerance-breakdown/