Background A hallmark feature of primary Sjögren's syndrome (pSS) is B cell hyperactivity, including presence of autoantibodies, aberrant presence of B cells and plasma cells in the salivary glands, elevated serum IgG levels and increased risk of lymphoma development. The mTOR pathway is essential for cell growth, survival and proliferation of B cells and mTOR inhibition has been shown to be effective in immune B cell suppression in transplant patients and in treatment of B cell lymphomas. Interestingly, in a pSS mouse model mTOR targeting inhibited lymphocytic infiltration in the lacrimal gland. However, mTOR activation in B cells has not been studied in pSS patients.
Objectives To study the mTOR pathway in B cells of pSS patients as a potential therapeutic target to inhibit B cell hyperactivity.
Methods Expression of mTOR pathway genes (MTOR, RPTOR, RICTOR, DEPTOR, AKT1, IGF1R, IGF1, PTEN) were assessed on an OpenArray platform in purified peripheral blood B cells and monocytes from pSS patients (n=12), non-Sjögren's sicca patients (n=17) and healthy controls (HC, n=9). Correlations with clinical parameters including lymphocytic focus score, ESSDAI and serum IgG levels were assessed. Flow cytometry analysis for B cell subset distribution was performed to assess potential effects of B cell subset distribution on gene expression differences. Culture experiments were performed to study inhibition of the mTORC1 pathway (phosphorylated S6, kinase activity downstream of mTORC1) in association with inhibition of B cell proliferation and IgG production by mTOR inhibition.
Results RPTOR and IGF1R expression were significantly increased in B cells from pSS patients (p=0.019 and p=0.018, respectively) and correlated with serum IgG levels (r=0.429, p=0.020, and r=0.462, p=0.012). Differences in expression of mTOR pathway genes were not found in monocytes. To indicate the mTOR signature a cumulative mTORC1 score was calculated consisting of Z scores (AKT1 + IGF1R + IGF1 + RPTOR + MTOR – PTEN – DEPTOR) which was significantly elevated in pSS (p=0.027), correlating with serum IgG levels (r=0.463, p=0.011). Frequencies of memory and naïve B cells did not differ between pSS patients and HC in this cohort (p=0.415). Activation of B cells in culture resulted in phosphorylation of S6, which indicates increased mTORC1 activity, in accordance with B cell proliferation and IgG production in both HC and pSS patients. Inhibition of mTOR by rapamycin decreased pS6 (n=4, n=2 HC, n=2 pSS, MFI 1400±335 vs 935±306, p=0.060), strongly reduced B cell proliferation (n=6, n=3 HC, n=3 pSS, 80.8±9.9 vs 19.1±15.8%, p<0.001), reduced IgG+ B cells (n=6, n=3 HC, n=3 pSS 39.5±15.5 vs 11.1±5.7%, p=0.001) and decreased IgG production (n=4, n=2 HC, n=2 pSS, 160±180 pg/mL vs 25.3±13.0 pg/mL p=0.060).
Conclusions Presence of an mTORC1 signature in B cells from pSS patients correlating with B cell hyperactivity indicates a role for mTORC1 in B cell activation in this disease. The fact that B cell proliferation and IgG production is effectively inhibited by rapamycin suggests that mTOR inhibition represents a potential therapeutic strategy for pSS.
Disclosure of Interest None declared