Aims Sclerosing lymphocytic lobulitis (SLL) of the breast is characterised by lymphocytic lobulitis, ductitis, vasculitis and dense keloidal fibrosis with epithelioid fibroblasts. However, the subsets of the infiltrating lymphocytes and their contribution to disease progression have not been fully explored.

Methods CD20, CD3, CD4, CD8 and regulatory T (Treg) lymphocytes were evaluated in the epithelial and vascular areas in SLL. The relationship between the lymphocyte subset in different regions and the degree of inflammation was analysed.

Results Lymphocytic infiltration was mainly located in peri-lobular, peri-ductal and peri-vascular areas. No significant differences between CD20 and CD3 lymphocytes were found in peri-epithelial areas. However, there were more intra-ductal/lobular epithelial CD3 than CD20 lymphocytes (p<0.001). For T lymphocyte subsets, more CD4 than CD8 lymphocytes were found in the peri-lobular/vascular regions (p≤0.026); but an opposite trend was seen in the intra-ductal/lobular regions (p<0.001). In the peri-lobular/vascular regions, generally, different lymphocyte subsets correlated with each other. Interestingly, in the peri-ductal region, only CD4 lymphocytes showed significant correlations with all other subsets (p≤0.020). Regarding their relationship with the degree of inflammation, significant positive correlations were observed for all subsets in peri-vascular/lobular regions (p≤0.045). Only regulatory T cells, but not the others, at the peri-ductal region showed significant correlation with the degree of inflammation at all three regions (p≤0.014).

Conclusions In addition to B lymphocyte subsets, T lymphocyte subsets could be involved differently in SLL. CD4 lymphocytes may have a pivotal role in recruiting other subsets to the inflamed site, and triggered the cascade of inflammatory changes resulting in fibrosis.