Here, we provide evidence that 17-DMAG, upon non-toxic concentrations, inhibited T cell proliferation and reduced percentages of Th1 and Th17 cells, which was associated with dampened Th1 (IFN-γ and TNF-α) and Th17 (IL-17) cytokine secretion. These results are in good agreement with previous studies reporting the capacity of Hsp90 blockers to inhibit proliferation of T lymphocytes ex vivo and to downregulate these proinflammatory T cell subtypes
Since Th1 and Th17 cells are essential to the development of various autoimmune diseases, treatment strategies which aim at blocking of uncontrolled activation of such effector cell populations are highly warranted
. In fact, pharmacological blockade of Hsp90 has been reported to be an effective treatment in rodent models of T cell-mediated autoimmune diseases, such as autoimmune encephalomyelitis
, rheumatoid arthritis
[7, 8], and systemic lupus erythematosus
[9, 10]. In addition, our research group recently demonstrated that, by downregulating T cell responses, treatment with Hsp90 inhibitors is also effective in mice with the experimentally induced autoimmune bullous disease epidermolysis bullosa acquisita
Although the main focus of our experiments was to study the impact of 17-DMAG on Th1 and Th17 subpopulations, we cannot rule out but also not support that 17-DMAG additionally exhibited suppressive activity on other T cell populations such as Th2 and regulatory T cells since Th2 cytokines released from anti-CD3 antibody-stimulated PBMCs were below the detection limit of our assay and secreted IL-10 and TGF-β1, cytokines associated with regulatory T cell function, were also undetectable or not significantly inhibited in our study, respectively. In this context, it is worth noting that there is evidence in the recent literature that Hsp90 inhibition can promote rather than inhibit regulatory T cells, further supporting an antiinflammatory mechanism of action of Hsp90 blockers in terms of T cell responses
Our current experiments further revealed that inhibition of T cells by 17-DMAG was associated with deactivation of NFκB and upregulation of Hsp70. While NFκB is a client of Hsp90 and one of the major transcription factors responsible for proliferation of T cells and their proinflammatory IFN-γ and IL-17 expression
[16, 17], Hsp70 is generally considered as a marker for effective Hsp90 inhibition and also regarded as potent antiinflammatory chaperone capable of inhibiting NFκB signaling pathways
Corticosteroids, which are widely used to treat patients with autoimmune diseases, mediate their immunosuppressive effects through cytosolic ligand-inducible glucocorticoid receptors. Inactive glucocorticoid receptors are associated with (co)chaperones, including Hsp90, which dissociate after their ligation, followed by nuclear translocation of these receptors and regulation of gene transcription
. The glucocorticoid receptor has been described as part of a T cell receptor-linked multiprotein complex containing Hsp90 and the nonreceptor tyrosin kinases Lck and Fyn, which is essential for T cell receptor-dependent Lck/Fyn activation. It has been previously shown that either treatment with dexamethasone or knocking down Hsp90 by Hsp90siRNA induces dissociation of this protein complex, resulting in abrogated T cell receptor signaling as a consequence of impaired Lck/Fyn activation
. Similar to this and other previous studies
[4, 13, 22], we could show in the current experiments that pharmacological blockade of Hsp90 was associated with inhibition of Lck activation in anti-CD3 antibody-stimulated PBMCs, a mechanism that could further account for the observed immunosuppressive effects of this treatment on T cells.
Considering that inhibition of both NF-kB function and proximal T cell receptor signaling by corticosteroids can be mimicked using Hsp90 inhibitors and that a novel generation of Hsp90 blockers with good tolerability has been reported in the field of cancer treatment
[4, 13, 22–25], it remains to be clarified in the future whether this class of drugs can potentially represent an effective alternative for corticosteroid therapy with a better side effect profile in patients with autoimmune diseases.
Together, our results further support the potential use of Hsp90 inhibitors in patients with autoimmune diseases where inappropriate activation of proinflammatory Th1 and Th17 subpopulations frequently occurs.