Mast cells in Hodgkin lymphoma or 'What's a nice cell like you doing in a tumour like this?'

Abstract: Mast cell (MC) accumulation around tumours is an old observation gaining new relevance due to the multifaceted nature of MCs and their many roles in immunity, beyond allergy. Knowledge about tumour specific recruitment of, and interactions with, MCs is needed to unravel the function of their presence.This study investigates the participation of mast cells in the tumourigenesis of Hodgkin lymphoma (HL), a tumour with many inflammatory features. We report that MC recruitment into HL lymphomatous tissue is possibly due to the production of CCL5/RANTES by malignant Hodgkin and Reed-Sternberg (HRS) cells. In addition, increased levels of IL-9, a cytokine implicated in mast cell heterogeneity and as an autocrine growth factor for HRS cells, were found in HL patient sera and correlate with negative prognostic factors. The ubiquitous expression of CD30 by HRS cells has been implicated in HL tumour development. In HL tissue MCs were found to be the predominant CD30 ligand (CD30L) expressing cells, and through CD30L/CD30 engagement they induced a proliferative response in HRS cells. This interaction proved to be bi-directional as it induced a degranulation-independent de novo synthesis of a specific set of chemokines in MCs, including IL-8. This novel trigger of MC activation is suggested to be of importance also in atopic dermatitis (AD) and psoriasis since increased numbers of CD30L and IL-8 positive MCs were detected along with increased expression of CD30.Data presented in this study supports a specific recruitment of MCs into HL tumours and co-operative interactions between HRS cells and MCs. Our identification of reversed signalling via CD30L as a novel MC trigger provides a mechanism behind leukocyte infiltration and chronic development in diseases associated with CD30 and MCs, such as HL, AD and psoriasis.