Deletion of Mucosa-Associated Lymphoid Tissue Lymphoma Translocation Protein 1 in Mouse T Cells Protects Against Development of Autoimmune Arthritis but Leads to Spontaneous Osteoporosis.

Abstract

Mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT-1) plays a crucial role in innate and adaptive immune signaling by modulating the threshold for activation of immune cells, including Treg cells. Therefore, MALT-1 is regarded to be an interesting therapeutic target in several immune-mediated diseases. The goal of this study was to examine the role of MALT-1 in experimental animal models of rheumatoid arthritis (RA). MALT-1 activation was assessed by measuring cleavage of the deubiquitinase CYLD in lymphocytes from mice with collagen-induced arthritis (CIA). Furthermore, the impact of MALT-1 deficiency on arthritis was evaluated in Malt1KO mice with CIA or with collagen antibody-induced arthritis (CAIA). T cell-specific MALT-1 deficiency was measured in mice with deletion of T cell-specific MALT-1 (Malt1Tcell KO ), and the time-dependent effects of MALT-1 deficiency were assessed in mice with deletion of tamoxifen-inducible T cell-specific MALT-1 (Malt1iTcell KO ). Bone density was determined in MALT-1-deficient mice using micro-computed tomography and femur-bending tests. Reconstitution of Treg cells was performed using adoptive transfer experiments. MALT-1 activation was observed in the lymphocytes of mice with CIA. T cell-specific MALT-1 deletion in the induction phase of arthritis (incidence of arthritis, 25% in control mice versus 0% in Malt1iTcell KO mice; P < 0.05), but not in the effector phase of arthritis, completely protected mice against the development of CIA. Consistent with this finding, MALT-1 deficiency had no impact on CAIA, an effector phase model of RA. Finally, mice with MALT-1 deficiency showed a spontaneous decrease in bone density (mean ± SEM trabecular thickness, 46.3 ± 0.7 μm in control mice versus 40 ± 1.1 μm in Malt1KO mice; P < 0.001), which was linked to the loss of Treg cells in these mice. Overall, these data in murine models of RA highlight MALT-1 as a master regulator of T cell activation, which is relevant to the pathogenesis of autoimmune arthritis. Furthermore, these findings show that MALT-1 deficiency can lead to spontaneous osteoporosis, which is associated with impaired Treg cell numbers.