The authors have analyzed 47 tumors of the central nervous system (11 glioblastomas, nine meningiomas, three medulloblastomas, 12 assorted primary neural tumors, and 12 brain metastases) for their content of macrophages. Cell suspensions were prepared by enzymatic digestion and macrophages were quantitated by IgGEAC rosette formation. Adsorption of sensitized indicator cells (EA) to sections of tumor was used as a measure to determine the distribution of IgGFc receptor-positive cells within the tumors and to serve as a control for selective release of IgGFc receptor-positive cells by enzyme digestion. The 11 glioblastomas had a mean macrophage content of 45% (range: 8% to 78%), the nine meningiomas had a mean of 44% (range: 5% to 81%), the three medulloblastomas a mean of 6% (range 2% to 15%), and the metastatic tumors a mean of 24% (range: 4% to 70%). Adsorption of EA demonstrated that IgGFc receptor-positive cells were distributed throughout the tumor mass, although different types of patterns were observed. There was an excellent correlation between the percent of IgGEAC positive cells in suspensions and the extent of EA adsorption to the tumor sections. Compared to systemic neoplasms, most nervous system tumors have a high macrophage content. It is possible that the high macrophage content of brain tumors is related to their immunogenicity, and may be a partial explanation for tha rarity of brain-tumor metastases.

Although the presence of lymphoreticular cells within tumors has been recognized for over 100 years, it is only within the last decade that the concept has arisen that standard histological examination techniques may lead to an underestimation of the true extent of tumor infiltration by lymphoreticular cells, and particularly by macrophages. The macrophage content of certain systemic tumors has been correlated with their immunogenicity and growth characteristics. Since the central nervous system is to some extent an "immunologically privileged site" and contains within it specialized reticuloendothelial cells called microglia, the authors determined the macrophage content of three rodent brain-tumor cell lines, and attempted to correlate this macrophage content with their immunogenicity and growth characteristics. Their findings indicate a direct correlation between the immunogenicity and macrophage content of these three neural tumor cell lines.