Association of Iba1-Positive Macrophages and B7-H3-Positive Tumor Cells with Tumor Growth Kinetics in WHO Grade II Meningioma: A Pilot Watch-and-Wait Cohort Study

Background/Objectives: WHO grade II meningiomas show highly variable growth rates that are difficult to predict using conventional histology. We investigated relationships between immune components within the tumor immune microenvironment (TIME) and explored their association with tumor growth kinetics. Methods: We retrospectively evaluated a small, preselected cohort of 15 patients with WHO grade II meningiomas who underwent initial “watch-and-wait” observation with serial preoperative MRI. Notably, this cohort represents a specific clinical subset—tumors deemed initially observable—and may not be representative of all WHO grade II meningiomas. Densities of Iba1-positive macrophages (TAMs) and B7-H3-positive tumor cells were quantified by digital pathology. The relative growth rate (RGR) was calculated from serial MRI. Results: A strong inverse correlation was observed between TAM density and B7-H3-positive tumor cell density (Spearman’s R = −0.921, p < 0.001). Individual immune components did not show significant linear correlations with RGR; on median-split stratification, high B7-H3-positive tumor cell density was associated with a nominally significant difference in RGR (p = 0.0428, unadjusted). Low TAM density showed a non-significant trend in the same direction. Conclusions: Our findings generate the hypothesis that WHO grade II meningiomas exhibit an inverse relationship between TAMs and B7-H3 and that macrophage-poor/B7-H3-high and macrophage-rich/B7-H3-low patterns may differ in preoperative growth behavior. These observations are derived from a small, preselected cohort of tumors managed with an initial “watch-and-wait” strategy and require validation in broader populations.