Ia antigen expression and IL-1 activity in murine tumour-associated macrophages

Tumour-associated macrophages (TAM) isolated from five murine sarcomas had a relatively high frequency of I-A⁺ cells, with mean values of 27% (mFS6), 52% (MN/MCA₁), 68% (N3), 62% (N4) and 98% (J3) for TAM compared to 12% for resident peritoneal macrophages. Expression of I-E in TAM was also high (19%) in the only sarcoma (N4) examined in this respect. Expression of I-A by TAM declined in culture but exposure to lymphokine supernatants maintained and increased the frequency of I-A⁺ cells in TAM. Transplantation of tumours into nude mice caused a marked decrease in the percentage of I-A⁺ TAM in the case of the N4 sarcoma (8% compared to 48%), whereas for the MN/MCA₁ sarcoma the diminution was only marginal (from 53 to 41%), TAM from murine sarcomas did not constitutively release appreciable levels of interleukin-1 (IL-1) activity. Upon stimulation with bacterial lipopolysaccharides or silica, TAM showed a limited capacity to produce and release IL-1 activity compared to peritoneal macrophages. Thus the expression of I-A antigens and IL-1-producing capacity are uncoupled in TAM from murine sarcomas. These properties of TAM could play an important role in the generation of anti tumour immunity and/or of suppressive T-cell circuits.