a 48hour shortterm treatment RAF Signaling Pathway with 17DMAG, ruling out possible compensatory mechanisms that may have occurred during longterm treatment. Germinal center B cells were also not affected by Hsp90 inhibitors in vivo. In contrast to the findings obtained with Hsp90 inhibition in transformed plasma cells,18 our data imply that in vivo antiHsp90 treatment does not lead to sufficient ER stress for initiation of the UPR and subsequent apoptosis of nonmalignant plasma cells. It is possible that Hsp90 exhibits diverse susceptibility toward its inhibitors in malignant and nonmalignant plasma cells. In fact, previous studies have established that Hsp90 produced by cancer cells is found within a multichaperone complex associated with high ATPase activity, which has a 100fold higher affinity for inhibitors than its free, uncomplexed form expressed in normal cells.
24 Another assumption is that either immunoglobulin is not the true client protein of Hsp90b1 or the immunoglobulinchaperoning function is redundant and can be compensated for by other immunoglobulininteracting ER chaperones such as GRP7825 and GRP170.26 A recent study found no intrinsic defect with B cells from Bcell–specific Hsp90b1null Fludarabine mice in terms of Bcell receptor expression, proximal signaling, immunoglobulin assembly and production, and plasma cell differentiation.27 Our data extend these previous observations and indicate that targeting Hsp90 has no impact on the survival of normal or autoreactive plasma cells in vivo.
Because T cells have recently been found to be required for both the production of autoantibodies and blistering in experimental EBA,5 we investigated whether prokaryotic these cells are affected by Hsp90 inhibition. Isolated draining lymph node cells from immunized mice stimulated with the Tcell–activating antiCD3 CD28 antibody revealed a dosedependent reduction of proliferation index when cocultured with Hsp90 inhibitors. Restimulation with recombinant type VII collagen was also suppressed, indicating that autoreactive T cells were targets of antiHsp90 treatment. These findings are in agreement with previous studies showing an inhibitory potential of Hsp90 blockade on activated T cells,12,14,28,29 including those of the autoreactive12,14 and alloreactive type.Although in the present study, autoreactive plasma cells were not depleted by antiHsp90 treatment, we found a suppressed serum autoantibody production in antiHsp90–treated mice.
There are 2 mutually nonexclusive explanations for this observation. The first is linked to the inhibitory effects of Hsp90 inhibitors on T cells, which are known to provide help to B cells. The second is based on the observation that the generation of a T cell–dependent, antigenspecific antibody response requires activation of Tolllike receptors in B cells, and on the increasing appreciation of the importance of these receptors in the pathophysiology of autoimmune diseases.30,31 Because HSP90b1 ablation in B cells has been recently shown to be associated with an attenuated antibody production in the context of Tolllike receptor stimulation,27 disruption of chaperoning Tolllike receptors rather than immunoglobulin assembly might have accounted for this suppressed autoantibody response after Hsp90 treatment.