Section 6 -

Extracellular Mediators in the Biology of Hodgkin Lymphoma (HL) Sibrand Poppema Department of Pathology & Laboratory Medicine University Medical Center Groningen, The Netherlands

Lymph nodes involved by HL generally contain only a minority of Reed-Sternberg (RS) cells suspended in an abundant inflammatory infiltrate, suggesting that immunological mechanisms contribute to HL pathogenesis. The inflammatory infiltrate in classical HL consists of varying numbers of predominantly CD4+ T cells, with a variable admixture of CD8+ T cells, B cells, plasma cells, eosinophils, histiocytes, neutrophils and mast cells, and usually comprises more than 99% of the tumor mass. RS cells secrete a number of cytokines, including IL5, 6, 9, and 13, but also immunomodulatory cytokines like TGF-β and IL-10 (Poppema). In particular, the cytokine IL-13 and also it receptor IL-13Ra1 were found to be highly expressed in Hodgkin cell lines and RS cells (Skinnider). This important cytokine promotes T-helper-cell differentiation to the Th2 phenotype, can indirectly promote immunosuppression, and can influence the survival and/or proliferation of B-cells by binding to IL-13Ra1 and activating STAT6. What causes the extensive infiltrate of lymphocytes and other inflammatory cells in HL? The explanation may be that RS cells produce and secrete high amounts of chemokine, in particular TARC ( T hymus and Activation Related Chemokine) and MDC ( M acrophage Derived Chemokine) , that attract cells expressing the CCR4 receptor, such as activated Th2 lymphocytes (van den Berg). And why is there no effective immune response against the tumor cells? HL infiltrating lymphocytes are anergic to stimulation with some mitogens and primary as well as recall antigens, but also suppress peripheral blood mononuclear cell (PBMC) responses. This appears to be caused by IL-10 secreting T cells as well as CD4+CD25+ regulatory T cells (Marshall ). The immunosuppressive effect of the HL infiltrating cells can be neutralized with anti IL-10, by preventing cell-to--cell contact and by anti CTLA-4. The lymphocytes in HL do not produce cytokines, such as IL-2, IL-4 and IFN-g, with primary (KLH) and recall (PPD) antigens and the mitogen ConA. However, when stimulated with PHA or with phorbolester (PMA) - ionomycin, the lymphocytes in HL are capable of producing these cytokines. Specifically, when the CD26 negative CD4 cells immediately surrounding the Reed-Sternberg cells were purified and stimulated with PMA ionomycin, these produced IL-4 and IFN-g. The potential to produce IL-4 was the reason why these cells were previously considered Th2 like. Absence of IL-2 production upon stimulation is also associated with anergy. The exact nomenclature of these cells is thus a matter of semantics. In addition to the IL-10 producing cells (Tr1) there are also TGFβ producing cells (Th3) present in the infiltrate. There are variations in the lymphocyte populations involved in different cases. It can be concluded that as an overall population, the infiltrating lymphocytes do not have Th1 type functions and are probably attracted into the tissues by chemokines TARC and MDC as CCR4 expressing Th2 cells. Although these cells do not spontaneously produce IL-2 or IL-4, they produce IL-10, despite not being fully activated, and therefore function as Tr1 cells. The major remaining question is: what causes the predominance of T cells with suppressor activity in Hodgkin Lymphoma? It appears that RS cells, although they have the genotype of B cells, execute a functional program with expression of molecules like CD40, CD80 and CD86, that is similar to that of antigen presenting cells, and is resulting in tolerance. Mechanisms include the production of immunosuppressive cytokines like IL-10, especially in Epstein-Barr virus positive cases, and IL-13, and of TGFβ, especially in Nodular Sclerosis cases. In addition, R- cells express FAS ligand, which induces cell death in FAS expressing activated T cells, while the RS cells themselves are protected by over-expression of cFLIP or infrequently by FAS mutation (Maggio). The relevance of these findings is that they may allow a better design of new treatment modalities. There are indications that the infiltrating cells in fact support the growth and survival of the RS cells, and therefore blocking chemokines like TARC and MDC to prevent the influx of T cells may be effective. Also interference with binding or signaling of IL-13 or other cytokines might be effective. On the other hand, blocking of the immunosuppressive signals, such as provided by IL-10 and TGFβ or the removal of the suppressor regulatory T cells, may enhance cytotoxic T-cell responses. References: Poppema S, van den Berg A.Interaction between host T cells and Reed-Sternberg cells in Hodgkin lymphomas. Semin Cancer Biol 2000;10:345-350 Skinnider BF, Elia AJ, Gascoyne RD, et al. Interleukin 13 and interleukin 13 receptor are frequently expressed by Hodgkin and Reed-Sternberg cells of Hodgkin lymphoma. Blood 2001;97:250-255. Van den Berg A, Visser L, Poppema S. High expression of CC chemokine TARC in Reed-Sternberg cells: A possible explanation for the characteristic lymphocytic infiltrate in Hodgkin 's disease. Am J Pathol 1999;154:1685-1691. Marshall NA, Christie LF, Munro LR et al. Immunosuppressive regulatory T cells are abundant in the reactive lymphocytes of Hodgkin Lymphoma. Blood 2004;103:1755-1762. Maggio E, van den Berg A, de Jong D et al. Low frequency of FAS mutations in Reed-Sternberg cells of Hodgkin lymphoma. Am J Pathol 2003;162:29-35.