Immune responses against differentiated thyroid carcinomas (DTC) have long been recognized. We aimed to investigate the role of immune cell infiltration in the progression of DTC.

We studied 398 patients – 253 with papillary and 13 with follicular thyroid cancers, as well as 132 with nonmalignant tissues.

Immune cell infiltration was identified using CD3, CD4, CD8, CD20, CD68 and FoxP3 immunohistochemical markers. In addition, we assessed colocalization of CD4 and IL‐17 to identify Th17 lymphocytic infiltration and colocalization of CD33 and CD11b to identify infiltration of myeloid‐derived suppressor cells (MDSC).

Immune cells infiltrated malignant tissues more often than benign lesions. The presence of chronic lymphocytic thyroiditis (CLT) concurrent to DTC, CD68+, CD4+, CD8+, CD20+, FoxP3+ and Th17 lymphocytes but not MDSCs was associated with clinical and pathological features of lower tumour aggressiveness and a more favourable patient outcome. A log‐rank test confirmed an association between concurrent CLT, tumour‐associated macrophage infiltration, and CD8+ lymphocytes and an increased in disease‐free survival, suggesting that evidence of these immune reactions is associated with a favourable prognosis.

Our data suggest that the tumour or peri‐tumoural microenvironment may act to modify the observed pattern of immune response. Immune cell infiltration and the presence of concurrent CLT helped characterize specific tumour histotypes associated with favourable prognostic features.