Finding the right help in the tumor microenvironment.

Tumor-infiltrating lymphocytes (TILs) contain substantial numbers of CD4+ T cells mediating pro- and antitumor functions. While CD4+ Tregs are well characterized and known to promote tumor immune evasion, the fingerprint of CD4+ Th cells that recognizes tumor antigens and serves to restrict disease progression has remained poorly discriminated. In this issue of the JCI, Duhen et al. analyzed tumors from patients with head and neck squamous cell carcinoma or colon carcinoma and identified a unique programmed cell death 1-positive, ICOS1-positive (PD-1+ICOS1+) subpopulation of CD4+ TILs highly enriched for the ability to recognize tumor-associated antigens. These cells localized proximally to MHC II+ antigen-presenting cells and CD8+ T cells within tumors, where they appeared to proliferate and function almost exclusively as Th cells. These potentially therapeutic Th cells can be monitored for patient prognosis and are expected to have substantial utility in developing personalized adoptive cell- and vaccine-based immunotherapeutic approaches for improving patient outcomes.