Background: Cell metabolism drives T cell functions, while platelets regulate overall CD4 + T cell immune responses. Objective: To investigate if platelets influence cell metabolism and thus regulate CD4 + T effector memory cell (Tem) responses. Methods: Human CD4 + Tem cells were activated with αCD3/αCD28 and cultured without or with platelets or platelet-derived mediators. Results: Polyclonal stimulation induced rapid and marked Th1 and Treg cell activation of CD4 + Tem cells. Platelet co-culture enhanced Th1 response transiently, while it persistently enhanced Treg cell activation of Tem cells, with an enhancement that plateaued by day 3. Platelet factor 4 (PF4) was the key platelet-derived mediator regulating CD4 + Tem cell responses, which involved cellular metabolisms as indicated by mass spectrometric analyses. PF4 exerted its effects via its receptor CXCR3, attenuated Akt activity, and reduced PGC1α phosphorylation, and resulted in elevations of PGC1α function and mitochondrial transcription factor A (TFAM) synthesis. The latter increased mitochondrial biogenesis, and subsequently enhanced Th1 and Treg responses. Consistent with these observations, inhibition of mitochondrial function by rotenone counteracted the enhancements by recombinant PF4, and TFAM overexpression by TFAM-adenovirus infection mimicked PF4 effects. Furthermore, increased mitochondrial mass elevated oxygen consumption, and enhanced adenosine triphosphate and reactive oxygen species production, which, in turn, stimulated Th1 (T-bet) and Treg (FoxP3) transcription factor expression and corresponding CD4 + T effector cell responses. Conclusions: Platelets enhance CD4 + T cell responses of Tem cells through PF4-dependent and Akt-PGC1α-TFAM signaling-mediated mitochondrial biogenesis. Hence, PF4 may be a promising intervention target of platelet-regulated immune responses.