Public-Data Reanalysis Links MS4A4A to M2-like Human Myeloid States and Supports a Predicted Four-Pass Transmembrane Fold

Background/Objectives: MS4A4A is associated with M2-like macrophage states, while the MS4A gene cluster modifies soluble TREM2 levels and Alzheimer's disease risk. We asked whether MS4A4A consistently marks the M2 side of human myeloid activation and whether AlphaFold supports a proposed MS4A4A-MS4A6A interaction. Methods: We reanalysed four public human datasets: bulk RNA-seq and ATAC-seq of primary monocyte-derived macrophages from independent three-donor cohorts, and single-cell RNA-seq atlases of healthy liver and severe COVID-19 blood. MS4A4A and an MS4A4A-MS4A6A complex were modelled with AlphaFold 3 and evaluated using pLDDT, predicted aligned error, and ipTM. Results: MS4A4A was higher in M2 (IL-4) than in M1 (IFN-gamma; + LPS) macrophages in all three donors (log2 fold change +2.68, adjusted P = 0.0013). Its promoter showed the highest mean accessibility in M2. MS4A4A was macrophage-enriched in liver and monocyte-enriched in blood, and was detected in 76.7% of M2-like versus 39.3% of M1-like liver macrophages, with the difference driven mainly by the proportion of positive cells. AlphaFold confidently modelled the four transmembrane helices (mean pLDDT 83.1), but the predicted MS4A4A-MS4A6A interface was not supported (ipTM 0.59). Conclusions: MS4A4A is consistently associated with the M2 side of human myeloid activation across independent transcriptomic, chromatin, and single-cell datasets. The findings are associative, and the proposed MS4A4A-MS4A6A interface remains an untested structural hypothesis.