Differences in the pathological, transcriptomic, and prognostic implications of lymphoid structures between primary and metastatic cutaneous melanomas

Background: While the prognostic role of tertiary lymphoid structures (TLS) has been well studied in solid cancers, the prevalence and impact of immature precursor lymphoid structures known as lymphoid aggregates (LA) remain unresolved in relation to the disease process. In this study, we examined characteristics and the prognostic utility of LA and TLS status in histological samples from patients with melanoma.

Methods: We assessed The Cancer Genomic Atlas-skin cutaneous melanoma digital slides and melanoma specimens from the University of Pittsburgh for the presence of LA and TLS using H&E staining, multiplex immunofluorescence (mIF) and transcriptomic analyses. Cox proportional hazard regression models were used to assess the prognostic value associated with the presence of lymphoid structures in melanomas.

Results: A total of 278 evaluable samples were analyzed and split into primary melanomas in skin (N=195) and metastatic melanomas involving skin/subcutaneous/soft tissue sites (N=83). 72% of tumor specimens contained histologically defined LA located in peritumoral (34%), intratumoral (5.6%) or stromal (6.1%) locations, with the remaining samples (54.3%) exhibiting LA in multiple locations. In contrast to LA which tended to form more commonly in primary melanoma samples, TLS with germinal centers predominantly formed in peritumoral (45.2%) or stromal (35.5%) locations in metastatic melanomas (p=0.02), with TLS observed in 11% of all melanoma specimens evaluated. mIF analyses revealed cellular heterogeneity of lymphoid structures, with CD20⁺ (B) cells present in nodule-shaped and stromal locations where they exhibited a high degree of colocalization with CD4⁺ and CD8⁺ T cells. A previously defined 12-chemokine gene expression score was significantly higher in samples with evidence of LA versus none (p<0.001), and samples without LA/TLS were enriched with pigmentation/neural network gene signatures. The presence of LA was significantly associated with tumor-free regional lymph node status (p=0.002). In multivariable analysis, after adjusting for age, sex, sample type, and stage, the presence of LA was associated with improved patient overall survival (OS) (HR=0.52, 95% CI 0.31 to 0.87, p=0.01).

Conclusion: Melanoma frequently contains LA, which tends to form in diverse locations in the tumor microenvironment in association with improved overall survival and tumor-free regional lymph node status in patients with primary disease.