Trp53 inactivation in the tumor microenvironment promotes tumor progression by expanding the immunosuppressive lymphoid-like stromal network

Inactivation of the tumor suppressor p53 through somatic mutations, observed in 50% of human cancers, is one of the leading causes of tumorigenesis. Clinical and experimental evidence also reveals that p53 mutations sometimes occur in tumor-associated fibroblasts, which correlate with an increased rate of metastases and poor prognosis, suggesting that p53 dysfunction in the tumor microenvironment (TME) favors tumor establishment and progression. To understand the impact of p53 inactivation in theTME in tumor progression, we compared the growth of subcutaneously inoculated B16F1 melanoma in p53^null and wild-type (WT) mice. Interestingly, tumor growth in p53^null mice was greatly accelerated, correlating with marked increases in CD11b ⁺Gr-1⁺ myeloidderived suppressor cells (MDSC), FoxP3 ⁺ regulatory Tcells, and a loss of effector function, compared with those in WT mice. This augmented immunotolerant TME in p53^null mice was associated with a marked expansion of a specialized stromal network in the tumor and spleen. These stromal cells expressed markers of fibroblastic reticular cells of lymphoid organs and were readily expanded in culture from p53^null, but not WT, mice. They produced high levels of inflammatory cytokines/chemokines and immunosuppressive molecules, thereby enhancing MDSC differentiation. Furthermore, they significantly accelerated tumor progression in WT mice when co-injected with B16F1. Together, our results show that tumor-stroma interaction in hosts with dysfunctional p53 exacerbates immunosuppression by expanding the lymphoid-like stromal network that enhances MDSC differentiation and tumor progression.