Mast cell activation leads to cisplatin-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a painful side effect of treating cancer with cisplatin. Cisplatin is a platinum-based chemotherapeutic that induces mast cell activation. Mast cells reside in close proximity of the epidermal layer, vasculature and nerve fibers in the skin. Tryptase and other noxious substances released by mast cells activate nociceptors via protease activated receptor -2 (PAR2) leading to pain. We hypothesized that cisplatin-induced mast cell activation leads to nerve injury and neuropathic pain. We used C57BL/6, wild-type (WT) and mast cell knockout (MC-KO) mice with a spontaneous c-kit "sash" mutation (Kit W-sh ) on a C57BL/6 background. Male and female mice were treated with cisplatin (i.p., 2.3 mg/kg/day), for 5 days of treatment followed by 5 days of saline (i.p.) for 2 cycles; or pre-treated with 100 mg/kg/day imatinib, an inhibitor of c-Kit and mast cell activation. Mechanical, cold, and thermal hyperalgesia were assessed at regular intervals. At day 18, cisplatin-treated mice showed a significant increase in mechanical hyperalgesia (p<0.001), cold hyperalgesia (p<0.05), and heat hyperalgesia (p<0.0001) compared to vehicle-treated mice. There were no changes in WT mice treated with imatinib or imatinib with cisplatin or in MC-KO mice treated with cisplatin for mechanical or thermal hyperalgesia, suggesting the involvement of mast cells in CIPN. Dorsal skin sections were co-stained with histone H3 (mast cell traps), FcεR1 (mast cell marker), and NF200 (nerve bundles). Cisplatin-treated mice showed activated mast cells surrounding nerve bundles, causing the expulsion of dense traps of citrullinated histones and podia extending into nerve fibers, causing nerve damage. The vehicle-treated mice showed intact, undisturbed thick nerve bundles without mast cell activation. Compared to vehicle, cisplatin-treated WT mice showed a significant increase in the number of non-degranulating and degranulating mast cells in dorsal (p<0.05) and toe skin (p<0.001, p<0.0001, respectively). We found a significant increase in cutaneous chymase (p<0.01), and tryptase (p<0.01), and plasma chymase (p<0.5), and tryptase (p<0.01) levels in cisplatin-treated compared to vehicle-treated WT mice. Human mast cells, HMC1.2, were incubated with 2, 5, and 10 µg/ml cisplatin or vehicle for 10 and 70 min, followed by analysis of chymase and tryptase in the conditioned medium. At the lowest dose of 2 µg/ml, cisplatin significantly stimulated the time-dependent release of chymase and tryptase compared to vehicle (p<0.01 for both). Together, our data show the novel phenomenon of mast cell traps upon cisplatin treatment leading to nerve injury, while tryptase released from mast cells may activate PAR-2 leading to the painful symptoms of CIPN. We speculate that the cotreatment of cisplatin with imatinib or other mast cell stabilizers such as cromolyn may ameliorate the painful symptoms of CIPN.