Clinicopathological Characteristics and Immune Microenvironment of Posterior Pituitary Tumours.

Aims: Posterior pituitary tumours (PPTs) represent a distinct group of low-grade neoplasms characterised by unique pathological features and immunohistochemical phenotypes. However, their precise immunological environment remains poorly understood, posing challenges to a deeper understanding of their biological behaviour and potential treatment implications. This study aimed to characterise the clinicopathological features and immune landscape of PPTs, focusing on the infiltration patterns of T cells and macrophages.

Methods: Sixteen PPTs were retrospectively analysed, including 10 pituicytomas (PC) and six spindle cell oncocytomas (SCO), using clinicopathological evaluation, immunohistochemical staining and multiplex immunofluorescence.

Results: Histopathological examination identified both classical and uncommon morphological characteristics, such as multinucleated giant cells, hemangiopericytoma-like vascular patterns, calcification and haemosiderosis. Immunohistochemical expression of programmed cell death ligand 1 (PD-L1) was rare, with only two cases showing focal positivity. Multiplex immunofluorescence for the analysis of the tumour immune microenvironment revealed distinct immune landscapes between PC and SCO. SCO exhibited a higher proportion of cytotoxic T lymphocytes (CTLs) and closer spatial interactions between macrophages and interferon (IFN)-γ-producing cells, reflecting a more robust antitumour immune response. In contrast, PC showed an increased proportion of exhausted CD8+ T cells, suggesting a more pronounced immunosuppressive microenvironment.

Conclusions: This study reveals distinct immune microenvironmental profiles between PC and SCO, with SCO exhibiting an immune-active phenotype, whereas PC is characterised by features of immune exhaustion. These differences underscore the potential for differential immunotherapeutic approaches, such as targeting immune exhaustion pathways in PC or enhancing CTLs activity in SCO.