The secreted tumor-associated antigen 90K is a potent immune stimulator.

Immunization of mice with conditioned media from human breast cancer cells yielded the monoclonal antibody SP-2, which recognized an antigen of approximately 90-95 kDa. This protein, designated 90K, was found to be present in the serum of healthy individuals and at elevated levels in the serum of subpopulations of patients with various types of cancer and AIDS. Here we report the primary structure of the SP-2 antigen and demonstrate its relationship to a family of proteins which carry a scavenger receptor cysteine-rich domain. Northern blot analysis of normal tissues, primary tumors, and tumor-derived cell lines indicates a broad expression spectrum of the 90K gene at widely varying levels. Functional characterization reveals stimulatory effects of 90K on host defense systems, such as natural killer cell and lymphokine-activated killer cell activity, and indicates that its immunostimulatory effects may be mediated through the induction of interleukin-2 and possibly other cytokines.