Expression of the B7 family molecules in acute myeloid leukemia (AML) has been demonstrated by independent clinical studies. Intriguingly, the expression of the most potent costimulatory molecules B7-2 (CD86) and B7-H2 (ICOS Ligand) on AML cells has been associated with poor prognosis and disease severity. Here, this phenomenon was modeled in vitro with the myeloid leukemia cell line HL-60, which is capable of differentiating through the FAB M2/M3 and M4/M5 immunophenotypes. These derivatives of HL-60 harbored a B7-2+ subpopulation and recapitulated the distribution of B7 ligands previously reported in primary AML cases. B7-2+ AML cells significantly contributed to T-cell responses. This costimulatory activity enabled helper (Th)-cell activation, proliferation, and production of Th1-associated cytokines. Conversely, even a short-term incubation with stimulated T cells resulted in upregulation of inhibitory B7-H1 (PD-L1) and B7-DC (PD-L2), and downregulation of stimulatory B7-H2 molecules on leukemia cells. Purified from iHL-60-T-cell co-cultures, these myeloid leukemia cells severely suppressed Th-cell responses specifically through the PD-1 pathway. In conclusion, Th-cell responses can be directly supported by B7-2+ leukemia subpopulations. However, this interaction can facilitate the acquisition of a suppressive character that may contribute to immune evasion in myeloid leukemia.