CD72 downregulation on DN2 B cells is associated with disease activity and resistance to rituximab in systemic lupus erythematosus.

Abstract

In SLE, expanded B cell subtypes like double-negative 2 (DN2) may harbour autoreactivity, which may be linked to impaired checkpoint regulation. We investigated checkpoint molecule CD72 on B cells, its clinical associations, and dynamic changes upon rituximab (RTX) treatment. Thirty SLE patients (26 with active disease) were studied. Seven received RTX treatment with sampling at baseline, 3 months and 6 months. B cell phenotypes were analysed using spectral flow cytometry, and clinical associations were evaluated. B cell receptor (BCR) signalling was studied through downstream phosphorylation in vitro. Patients with active SLE showed increased frequencies of CD72-negative B cells in switched memory (SWM), activated naïve (aNAV), DN2 and plasmablast (PB) subsets compared with healthy controls (HCs). CD72-negative DN2 cells (CD27-IgD-CD21-CD11c+) were elevated in LN patients. These cells expressed higher CD95 and lower CD20 compared with HCs and canonical SLE DN2. Upon BCR stimulation, lupus CD72-negative SWM and DN2 B cells displayed increased pSYK phosphorylation compared with their CD72-positive counterparts and the overall cell population. CD72-negative DN2 frequencies associated positively with SLEDAI-2K and inversely with C3 levels and were increased in anti-dsDNA-positive patients. After RTX treatment, the remaining DN2 cells were primarily CD72-negative at 3 months. The DN2 B cell subset have been associated with autoimmunity. We showed that DN2 B cells in SLE have reduced CD72 expression, which is associated with anti-dsDNA positivity, complement consumption, and enhanced BCR downstream signalling. The relative resistance of CD72-negative B cells to RTX indicates that an impaired checkpoint programme in lupus B cells may potentially contribute to disease relapse. These findings require validation in larger cohorts.