# Lymphoid Aggregates in Canine Cutaneous and Subcutaneous Sarcomas: Immunohistochemical and Gene Expression Evidence for Tertiary Lymphoid Structures

## TL;DR

This study shows that large lymphoid aggregates in canine sarcomas resemble human tertiary lymphoid structures, which are linked to better cancer outcomes.

## Contribution

The study provides the first evidence that large lymphoid aggregates in canine sarcomas meet the criteria for tertiary lymphoid structures.

## Key findings

- Large lymphoid aggregates in canine sarcomas show higher CD20 and CD3 mRNA expression compared to adjacent tumor tissue.
- These aggregates also exhibit chemokine RNA profiles consistent with tertiary lymphoid structures.
- Plasma cells and high endothelial venules were found in lymphoid aggregates but not in control tissue.

## Abstract

Canine cutaneous/subcutaneous soft‐tissue sarcomas (STS) are diversely derived mesenchymal neoplasms with a risk of recurrence and/or metastasis depending on the extent of surgical excision and histologic grade. Lymphoid aggregates (LAs) are often described in these tumours but not characterised. In humans, LA characterised as tertiary lymphoid structures (TLSs) improve the prognosis of many tumours, including sarcomas. We sought to determine if LA meeting a size criterion (> 700 cells) in canine sarcomas met the criteria of TLS and the overall prevalence of LA of any size. RNA expression in large LAs versus aggregate‐adjacent sarcoma tissue (AAS) was measured in laser capture microdissected tissue and compared to curl‐derived RNA from aggregate‐free sarcomas and lymph nodes. CD3, CD20, MUM‐1 and PNAd expressions were measured using immunohistochemistry. CD20 and CD3 mRNA were more highly expressed in LA versus AAS (13.8 fold, p = 0.0003 and 2.3 fold, p = 0.043). This was supported by the IHC findings. The large LAs were also enriched in chemokine RNA expression characteristic of TLS (CXCR5 5.8 fold, p < 00001, CCL19 3.68 fold, p = 0.0209, CCL21 6.87 fold, p = 0.0209 and CXCL13 2.68 fold, p = 0.0924). Plasma cells and high endothelial venules were identified in LA containing tumours but not in control tissue. Large LAs were present in 12% of tumours, and LA of any size in 30%. We conclude that large LAs in canine STS are consistent with TLS.