It seems many types of cancer can evolve resistance to treatment with a single type of immunotherapy. Similar to how HIV quickly evolved resistance to treatment by one ARV (antiretroviral).
But HIV can be kept in check by a cocktail of 3 ARVs simultaneously. And there's some recent research from Mass General where they managed to eradicate brain tumors in mice using 3 different immunotherapies simultaneously: https://advances.massgeneral.org/neuro/journal.aspx?id=1263
> Dr. Curry's group describes "a likely oversimplified, but reasonable" summary of the mechanism of the triple therapy: GVAX expands the number and diversity of activated tumor-specific T cells and increases the number of intratumor CD8+ T cells, while PD-1 inhibition further invigorates the effects of those mobilized cells. Meanwhile, agonist anti-OX40 continues to skew systemic and tumor microenvironments toward Th1 immunity, suppress regulatory T cells and prevent T cell exhaustion.
Hopefully this is a way forward for treating humans too!