The immune microenvironment in typical carcinoid lung tumour, a brief report of four cases

Pulmonary typical carcinoid (TC) is a low-grade, rare lung cancer of neuroendocrine origin. Currently, there is very little information available about the immune cell composition in TC tumours. Here, we analysed by flow cytometry resected tumours from four never-smoker female patients with TC. Twelve distinct immune cell types were identified in TC tumours. The most abundant immune cells were CD8⁺ T cells, CD4⁺ T cells, B cells and macrophages, which represented 19.8%, 17.7%, 11.5% and 11% of all tumour-infiltrating CD45⁺ leucocytes, respectively. Natural killer (NK) cells (8.8%) and neutrophils (3.9%) were also common. Three types of dendritic cells (DCs) were identified (plasmacytoid DCs, CD1c  DCs, and CD141  DCs) which together constituted 1.4% of all immune cells in TC tumours. Small populations of basophils (1.2%), mast cells (0.8%) and eosinophils (0.6%) were also present. Notably, the percentage of leucocytes (of all living cells) was much lower in TC tumours compared to high-grade non-small cell lung cancer (NSCLC) tumours and also compared to non-cancerous lung tissue. We conclude that TC tumours are relatively non-inflammatory, although the immune landscape was found to be very complex.