Impact of tumor-associated macrophages in LH_BETAT_AG mice on retinal tumor progression: Relation to macrophage subtype

PURPOSE. To determine the distribution of tumor-associated macrophages (TAMs) during retinoblastoma tumor development, examine the contribution of bone marrow-derived TAMs in retinoblastoma tumors, and evaluate the supportive role of TAMs in tumor growth in a transgenic retinoblastoma mouse model. METHODS. The time course of macrophage infiltration in transgenic retinoblastoma tumors was assessed by immunohistochemistry at different time points in tumorigenesis. The origin of TAMs in transgenic retinoblastoma tumors was determined by transplanting 10⁷ bone marrow cells from green fluorescent protein (GFP)-positive 16-week-old mice into age-matched, irradiated LH_BETAT_AG mice via tail vein injections. Macrophage depletion was performed by subconjunctival (SC) delivery of liposomal clodronate. RESULTS. The density of TAMs increased from 4 to 12 weeks of age in mice with small to medium tumors (P = 0.037) and remained stable in the later stages of disease (i.e., 16 weeks old with large tumors; P = 0.20). In 16-week-old mice, 38% (2.5 ± 3.2 cells per 400× high-power field) of TAMs were GFP-positive, bone marrow-derived macrophages. Total TAM depletion was associated with a significant decrease in the expression levels of MMP-9 (P = 0.014) and mature vessels (P < 0.001) and a nonsignificant decrease in the density of neovessels (P = 0.94). The density of M2-polarized TAMs did not change significantly after TAM depletion (P = 0.68). After M1-polarized TAM depletion, the tumor burden increased (P = 0.056). CONCLUSIONS. This work extends understanding of the complex role that macrophages play in retinoblastoma. Macrophage modulation in the tumor microenvironment is a critical factor in retinoblastoma tumor progression.