Gr-1(+) myeloid cells derived from tumor-bearing mice inhibit primary T cell activation induced through CD3/CD28 costimulation

Activation of T cells is a necessary step in the development of a specific antitumor immune response, In the present study, we evaluated the ability o f Gr-1(+) myeloid cells, derived from the bone marrow or spleen of tumor-be aring mice, to inhibit CD3/CD28-mediated T cell activation. Using flow cyto metry, we found that growth of a murine colon carcinoma (MCA-26) induces a significant increase in the number of Gr-1(+) and Gr-1(+)/Mac-1(+) myeloid cells in both bone marrow and spleen of the tumor host. The proliferative r esponse of T cells was dramatically decreased when naive T cells were activ ated by anti-CD3 and anti-CD28 Abs in the presence of a myeloid-enriched ce ll fraction derived from spleen or bone marrow of tumor-bearing mice vs the bone marrow of naive mice. Reversal of the inhibitory effect could be achi eved by adding a combination of MnTBAP (manganese [III] tetrakis [4-benzoic acid]) porphyrin and L-NMMA (N-G-monomethyl-L-arginine), a superoxide dism utase mimetic and inducible NO synthase inhibitor, respectively, or by depl etion of the Gr-1-positive cells. IFN-gamma, which is endogenously produced by CD3/CD28-stimulated naive T cells, is involved in induction of the inhi bitory activity of myeloid cells. Importantly, when T cells pre-activated w ith anti-CD3 Abs were used as responder cells, the bone marrow- or spleen-d erived Gr-1(+) myeloid cells were unable to suppress CD3/CD28-induced T cel l proliferation. Our findings suggest that one mechanism by which an increa sed number of immune suppressive Gr-1(+) cells can induce T cell unresponsi veness or immune tolerance in tumor hosts could be through peroxynitrite pr oduction upon primary T cell activation.