compensatory mechanisms of plasma cells after longterm treatment with Hsp90 inhibitors, splenic plasma cells were also analyzed 48 hours after the start of treatment with 17DMAG. MEK Signaling Pathway No depletion of plasma cells was achieved after shortterm treatment . Comparable results were obtained with TCBL145 . We also investigated whether Hsp90 inhibitors may have an impact on autoreactive plasma cells. Similar to normal plasma cells, immunohistochemical studies revealed that the numbers of CD138 mCVIICspecific plasma cells from draining lymph nodes were not significantly altered by 17DMAG after longterm or shortterm treatment or treatment with TCBL145 .
The results of the present study indicate that in contrast to malignant plasma cells, normal or autoreactive plasma cells do not present targets of Hsp90 inhibitors in vivo, and that the efficacy of antiHsp90 Cyclophosphamide treatment is at least in part mediated by immunosuppressive functions on Tcell responses in autoimmunity to type VII collagen. 17DMAG and TCBL145 effectively suppressed the development of EBA disease when administered before the appearance of clinical signs and induced clinical recovery when applied to mice that were already diseased. Compared with control mice, animals treated with Hsp90 inhibitors showed a reduced dermal inflammatory infiltrate at the dermalepidermal junction and lower levels of circulating autoantibodies against the basal membrane zone. Our data are in agreement with previous observations in animal models of other autoimmune diseases, including autoimmune encephalomyelitis, 12 rheumatoid arthritis,13 and systemic lupus erythematosus– like autoimmune disease,14 in which inhibitors of Hsp90 affected inflammatory disease pathways and efficiently improved the clinical course.
The mechanisms of action by which Hsp90 inhibitors led to clinical improvement in these mouse models included reductions in: maturation of dendritic cells,14 populations of antigenpresenting cells,14 activated T and B cells,12,14 and cytokine production.12,13 However, the effects of antiHsp90 treatment tissues on autoantibodyproducing plasma cells have not yet been studied. This is an important issue to address because Hsp90b1 is believed to be one of the key downstream chaperones in the ER that controls the ER UPR, which is increasingly being shown to modulate plasma cell function.
15 In vitro, both proteasome and Hsp90 inhibition have been linked to UPRmediated death of malignant plasma cells in multiple myeloma caused by the buildup of misfolded immunoglobulins within the ER.17,18 It was later shown that normal plasma cells are also hypersensitive to proteasome inhibition because of their extremely high amount of protein biosynthesis. Bortezomib depletes normal and autoimmune plasma cells from bone marrow and spleen in vivo via activation of the UPR and protects mice with lupuslike disease from nephritis.19 In contrast, we found no effect of antiHsp90 treatment on the survival of normal or autoreactive plasma cells in vivo. The numbers of B220 from spleen and bone marrow and type VII collagen–specific plasma cells from draining lymph nodes after the 6week treatment with either 17DMAG or TCBL145 were comparable to plasma cell counts of mice treated with vehicle. Splenic plasma cells were also not depleted .