Fumarate hydratase-deficient renal cell carcinoma: high intratumoral and peritumoral CD4-positive T cell infiltration density and high PD-L1 expression.

Background: Fumarate hydratase (FH)-deficient renal cell carcinoma (RCC) represents a rare and aggressive subtype of RCC, closely associated with hereditary leiomyomatosis and renal cell cancer syndrome (HLRCC). The limited understanding of this disease presents significant clinical challenges in its management. To date, robust evidence supporting the efficacy of systemic therapies for FH-deficient RCC remains scarce.

Objective: The objective of this study is to assemble a relatively large single-center cohort of FH-deficient RCC, focusing on clinical, pathological, and immune cell infiltration characteristics, particularly CD4-positive (CD4⁺) and CD8-positive (CD8⁺) T-cell infiltrates. This aims to enhance our understanding of FH-deficient RCC and provide evidence-based support for optimizing its systemic treatment strategies.

Methods: We have retrospectively reviewed clinicopathologic and genetic prognostic data of patients operated on for renal tumor between January 2013 and June 2023 at Peking University First Hospital. Additionally, we employed multiplex immunofluorescence to evaluate the expression profiles of T cells within the tumor-infiltrating microenvironment of these patients.

Results: This study analyzed 27 patients (median age: 39.3 years; range 16-70; M: F = 14:13) with 31 renal tumors, including two multifocal cases. Histopathological evaluation revealed 25 high-grade tumors (WHO/ISUP G3-4) and two low-grade tumors (one with focal high-grade features and one entirely low-grade). Immunohistochemistry demonstrated universal strong 2SC positivity in all cases, while GATA3 expression (7 cases) was largely focal. Genomic profiling of 23 patients identified 18 germline and 3 somatic FH mutations, with two cases lacking FH alterations; MSI-L was detected in two tumors (others MSS). Multiplex immunofluorescence of 19 FH-deficient renal cell carcinomas revealed elevated PD-L1 expression (63.16% tumor cells) and distinct immune infiltration patterns: CD4⁺ T cell density exceeded CD8⁺ cells overall (1,125 vs. 336/mm², P = 0.005), with higher T cell accumulation at tumor margins (CD4⁺: 1,431/mm²; CD8⁺: 332/mm²) versus centers (CD4⁺: 911/mm²; CD8⁺: 164/mm²; P_CD4=0.005, P_CD8=0.017). CD4⁺ dominance persisted across all regions, suggesting a unique immunophenotypic signature in FH-deficient tumors. In FH-deficient RCC, only PD-L1 expression significantly predicted survival: PD-L1-positive patients had prolonged OS (median NR vs. 29.73 months in PD-L1-negative; P = 0.03).

Conclusion: This analysis highlights the spatial heterogeneity of T-cell infiltration in FH-deficient RCCs and suggests a potential role for PD-L1-targeted therapies in this subset of tumors. These findings suggest a distinct spatial distribution of immune cell subsets within FH-deficient RCCs, with a pronounced enrichment of CD4-positive cells relative to CD8-positive cells, particularly at the tumor margin. This pattern may have important implications for understanding the tumor immune microenvironment and developing targeted immunotherapies for this subtype of RCC.