Immune structures of the greater omentum and their role in cancer metastasis

Omental malignant metastases are one of the challenging issues in oncology. The article provides a review of recent studies on the structure and functions of immune compartments of the greater omentum and their characteristic features that promote or inhibit tumor dissemination. The cellular composition of lymphoid nodules and milky spots is described, and functional and phenotypic properties of macrophages and lymphocytes are shown. Unique subpopulations of immunocompetent cells typical of this particular organ, as well as produced cytokines, are characterized. Particular attention is paid to the visceral adipose tissue surrounding immunocompetent cells and its effect on their functions. Analysis of the literature data has revealed a dual role, i.e. both protective and immunosuppressive one, of lymphoid structures of the greater omentum. The former is apparently associated mainly with a response to bacterial pathogens, while the latter is realized in cancer metastasis. The article focuses on immunological mechanisms that create local conditions for the growth and development of metastases, in particular, proinflammatory cytokines, chemokines, growth factors secreted by immune, tumor and mesothelial cells, and on the importance of the surrounding visceral adipose tissue for this process. The multidirectional prognostic significance of some local cellular and cytokine factors in cancer metastasis to the omentum and peritoneum is demonstrated. Possible approaches to treatment involving immunotherapy should be aimed at both elimination of tumor cells and overcoming the immunosuppressive environment. In this regard, reprogramming of macrophages, correction of hypoxic microenvironment, and the search for new control points seem promising.