IDO1 and neoplasm: On the other hand, release of CXCL8, CCL2, CXCL5, and CXCL12, CCL22, and CCL28 attracts Tregs (24); (iii) Arrest of clonal expansion of CTLs, mediated by tumor cell, dendritic cell, and MDSCs secretion of indolamine-2,3-oxygenase (IDO), which induces degradation of tryptophan, an indispensable molecule for CTLs growth and production of Granzyme B (25); (iv) Impaired expression of human lymphocyte antigen-I (HLA-I) and other molecules involved in the antigen presentation machinery, leading to reduced tumor antigen recognition, impaired immune response, and worse prognosis (26, 27).