Cancer cells have a remarkable ability to neutralize cells that would normally eliminate them. Aggressive tumors, in particular, often stimulate checkpoint inhibitors of the T cell activation cascade, rendering tumor infiltrating lymphocytes (TIL) ineffective. Current therapeutic strategies focus on removing tumor-initiated inhibition to reactivate T cells: Keytruda® and Yervoy®, which inhibit programmed death-ligand 1 (PD-L1) and cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) signaling, respectively, have demonstrated significant clinical success. Recently, collaborators at The Perelman School of Medicine at The University of Pennsylvania (UPenn) took this approach one step further: they evaluated whether using both kinds of checkpoint inhibitors in combination with radiation might provide more robust and effective therapy for treating patients with melanoma (Twyman-Saint Victor CT et al. 2015).
The UPenn investigators performed clinical trials to test the efficacy of combining anti-CTLA4 with tumor radiation for treating metastatic melanoma. Although this treatment stalled tumor progression in a few patients and lowered overall mortality, most patients did not respond. To understand the mechanisms for this resistance, C57BL/6J (000664) mice were engrafted with the syngeneic melanoma cell line B16-F10 and were given anti-CTLA4 and radiation treatments similar to those administered to the patients in the clinical trial. The proportions of responding and non-responding tumors in the engrafted mice mirrored those observed in the patient trial. Further, in cells from resistant mouse tumors, a compensatory increase in PD-L1 was noted, suggesting that the tumors could directly dampen T cell immune responses. Enhanced PD-L1 signaling in these tumors resulted in T cell exhaustion, as measured by decreased proliferation and lower cytotoxic protein production. Interestingly, anti-PD-L1 treatment in combination with anti-CTLA4 and radiation enhanced the therapeutic response of resistant tumors and resulted in increased overall survival in comparison to any other combination therapy (Figure 1).
Each component of this three-part therapy targets a distinct pathway in T cell biology. First, anti-PD-1 monoclonal antibody treatment, which blocks the PD-1/PD-L1 T cell checkpoint interaction, increases cytotoxic T lymphocyte (CTL) numbers and both anti-tumor cytokine and cytotoxic enzyme production. Second, inhibition of the T cell checkpoint protein CTLA4 increases overall CTL activity by decreasing anti-inflammatory T regulatory cells. Finally, radiation therapy increases CTL repertoire diversity that may enhance tumor antigen recognition. Collectively, this combination therapy facilitates improved tumor recognition and responses by cytotoxic T cells (Figure 2).
The UPenn researchers also defined biomarkers for tumor responsiveness that could be monitored in the peripheral blood of mice and by extension, in human patients. By measuring proliferating and cytokine-producing CTLs and the CTL to regulatory T cell ratios in their engrafted and treated mice, the investigators demonstrated that tumor response to therapy was 83% predictive. Translating this to patients on the original radiation and CTLA4 clinical trial, they further showed that responding and non-responding patients could have been predicted using similar measures, including PD-L1 expression. As a whole, the UPenn study suggests that triple therapy with multiple immune cell checkpoint inhibitors and radiation is a more effective approach for treatment metastatic melanoma than either treatment alone.