The Immunosuppressive Niche of Soft Tissue Sarcomas is Sustained by Tumor Associated Macrophages and Characterized by Intratumoral Tertiary Lymphoid Structures.

Clinical trials with immune checkpoint (IC) inhibition in sarcomas have demonstrated minimal response. Here, we interrogated the tumor microenvironment (TME) of two contrasting soft tissue sarcomas (STS) - Rhabdomyosarcomas (RMS) and Undifferentiated Pleomorphic Sarcomas (UPS) - with differing genetic underpinnings and responses to IC inhibition in order to understand the mechanisms that lead to response.Utilizing fresh and Formalin Fixed Paraffin Embedded (FFPE) tissue from patients diagnosed with UPS and RMS, we dissected the TME by using immunohistochemistry (IHC), flow cytometry, and comparative transcriptomic studies.

Our results demonstrated both STS subtypes to be dominated by tumor associated macrophages (TAMs) and infiltrated with immune cells that localized near the tumor vasculature. Both subtypes had similar T-cell densities, however their in-situ distribution diverged. UPS specimens demonstrated diffuse intratumoral infiltration of T-cells, while RMS samples revealed intratumoral T-cells that clustered with B-cells near perivascular beds, forming tertiary lymphoid structures (TLS). T-cells in UPS specimens were comprised of abundant CD8⁺ T-cells exhibiting high PD-1 expression, which might represent the tumor reactive-repertoire. In RMS, T-cells were limited to TLS, but expressed ICs and immunomodulatory molecules which, if appropriately targeted, could help unleash T-cells into the rest of the tumor tissue.

Our work in STS revealed an immunosuppressive TME dominated by myeloid cells, which may be overcome with activation of T-cells that traffic into the tumor. In RMS, targeting T-cells found within TLS may be key to achieve anti-tumor response.