Tumor microenvironment in primary central nervous system lymphoma (PCNSL).

Primary central nervous system lymphoma (PCNSL) is one of the rare lymphomas limited to the central nervous system. With the availability of immunotherapy, the tumor microenvironment (TME) attracts much attention nowadays. However, the systematic studies on the TME of PCNSL are lacking. By reviewing the existing research, we found that the TME of PCNSL is infiltrated with abundant TAMs and TILs, among which cytotoxic T cells (CTLs) and M2-polarized macrophages are principal. However, the counts of immune cells infiltrated in the TME of PCNSL are significantly lower than systemic diffuse large B-cell lymphoma (DLBCL). In addition, PCNSL can attract the infiltration of immunosuppressive cells and the loss of HLA I/II expression, overexpress inhibitory immune checkpoints, and release immunosuppressive cytokines to form an immunosuppressive TME. The immunosuppressive effect of TME in PCNSL is significantly stronger than that in systemic DLBCL. These characteristics of TME highlight the immunosuppression of PCNSL.