Using a unique resource of samples from a controlled human malaria infection (CHMI) study, we identified a novel population of CD4⁺ T cells whose frequency in the peripheral blood was inversely correlated with parasite burden following P. falciparum infection. These CD4⁺ T cells expressed the multifunctional ectoenzyme CD38 and had unique features that distinguished them from other CD4⁺ T cells. Specifically, their phenotype was associated with proliferation, activation and cytotoxic potential as well as significantly impaired production of IFN-γ and other cytokines and reduced basal levels of activated STAT1. A CD38⁺ CD4⁺ T cell population with similar features was identified in healthy uninfected individuals, at lower frequency. CD38⁺ CD4⁺ T cells could be generated in vitro from CD38^- CD4⁺ T cells after antigenic or mitogenic stimulation. This is the first report of a population of CD38⁺ CD4⁺ T cells with a cytotoxic phenotype and markedly impaired IFN-γ capacity in humans. The expansion of this CD38⁺ CD4⁺ T population following infection and its significant association with reduced blood-stage parasite burden is consistent with an important functional role for these cells in protective immunity to malaria in humans. Their ubiquitous presence in humans suggests that they may have a broad role in host-pathogen defense.

ClinicalTrials.gov clinical trial numbers ACTRN12612000814875, ACTRN12613000565741 and ACTRN12613001040752