Dendritic cell density and activation status of tumour-infiltrating lymphocytes in metastatic human melanoma: possible implications for sentinel node metastases

Nodal deposits of melanoma may present many years after resection of the primary tumour, implying initial suppression of tumour growth with subsequent immune escape. Using immunocytochemical techniques on frozen sections, the cellular types and activation status of infiltrating cells within a series of 19 clinically apparent nodal metastases of melanoma were studied. Infiltrating cells were assessed using a semiquantitative grading system. Macrophages (CD68+) and T-lymphocytes (CD3+) (including both CD8+ and probably also CD4+ T-cells) were the predominant cells infiltrating the tumours. B-lymphocytes (CD20+) were generally present in low numbers. CD1a+ putative dendritic cell density and expression of the early lymphocyte activation markers interleukin-2 receptor alpha (IL2Ralpha) and CD69 was low. However, greater evidence of intermediate lymphocyte activation (CD38) was identified. Expression of interleukin-2 (IL2) by tumour-infiltrating cells was not detected. The paucity of staining for IL2 and IL2Ralpha, with greater expression of CD38 by infiltrating cells, suggests that the usual pathways of lymphocyte activation via IL2 were bypassed or impaired within the lymph node metastases. Low numbers of CD1a+ putative dendritic cells may result in reduced effector cell activation. These findings provide evidence to support the hypothesis that antitumour immune responses within clinically involved lymph nodes are reduced in metastatic melanoma. This also has possible implications for micrometastases to the sentinel lymph node.