Introduction: The tumour microenvironment significantly influences tumour behaviour. Little is known about the pituitary adenoma microenvironment. AIP mutation positive (AIPpos) patients develop often aggressively growing pituitary tumours and the study of their microenvironment might identify factors leading to this aggressive phenotype which could help predict tumour behaviour and identify novel therapeutic targets.
Aim: The aim of this study was to extensively investigate the tumour microenvironment and discover its potential role in AIPpos tumourigenesis.
Methods: We have evaluated the expression of macrophages (CD68), T-reg cells (FOXP3), cytotoxic T cells (CD8), memory T cells (CD45RO) and vascular structures (CD31) as well as CCL5 and Fli-1 by immunohistochemistry on AIPpos and sporadic tumours. Stable AIP-knockdown pituitary somatotroph cells (GH3_AIP_KD) was used to study tumour-stromal cross-talk. Pituitary samples of a pituitary-specific AIP-knockout mouse model (AipFlox/Flox;Hesx1Cre/+) were also studied for macrophage cells.
Results: Immunohistochemical analysis revealed significantly increased levels of CD68+ macrophages (P=0.01), FOXP3+ T-reg cells (P=0.009) and vascular elements (P=0.02) in AIPpos tumours compared to sporadic somatotrophinomas. No differences were found in CD8 and CD45RO staining. GH3_AIP_KD cells attracted increased macrophage migration compared to non-targeting controls via the CCL5/CCR5 pathway. AIPpos tumours also showed higher levels of expression of pro-inflammatory cytokine CCL5 (P=0.001) and its regulatory transcription factor Fli-1 (P=0.003). Furthermore, the level of macrophage infiltration was increased in the pituitary-specific AIP-knockout pituitary samples, similar to human AIPpos tumours.
Conclusions: Using molecular markers, in vitro and in vivo models we have revealed that AIPpos tumours have a unique microenvironment which is strikingly different compared to sporadic somatotrophinomas. It appears that AIP deficiency in pituitary cells drives aggressive tumourigenesis partially via increasing immune cell infiltrates. Therefore, new treatments targeting the tumour microenvironment may have potential for the management of patients with aggressive AIPpos pituitary tumours.