Single-cell profiling separates immune states across breast cancer subtypes
Breast cancer does not present as a single immune disease. In this PLOS One analysis, single-cell RNA sequencing was used to compare the immune microenvironment across HER2-positive, triple-negative breast cancer, and ER-positive tumors, with attention centered on T cells, macrophages, and B cells. The resulting atlas shows that each molecular subtype carries a different immune architecture, and those differences reach beyond simple cell counts. Functional state, transcriptional program, and intercellular signaling all shift in subtype-specific ways.
The most striking pattern involved CD8+ cytotoxic T cells. These cells were more abundant in TNBC, yet their biology was not uniformly favorable. Greater abundance did not translate into fully effective antitumor activity; instead, these cells showed signs of dysfunction. In practical terms, a tumor can recruit more cytotoxic lymphocytes and still evade immune pressure if those lymphocytes are exhausted, impaired, or locked into a less effective activation state.
Cytotoxic T cells were expanded in TNBC but functionally compromised
The TNBC immune environment appears especially rich in cytotoxic T-cell infiltration, but the transcriptomic signature described here suggests a split between presence and performance. This is one of the central advantages of single-cell analysis: it distinguishes a cell’s identity from its state. A bulk assay could easily miss that distinction and overestimate the strength of a so-called inflamed tumor.
A dysfunctional cytotoxic compartment carries direct therapeutic implications. Immune checkpoint strategies depend on T cells that remain capable of reactivation, clonal expansion, and target killing. If the dominant CD8+ population in TNBC is already in a dysfunctional state, the tumor may require a different immunologic strategy than a subtype with fewer infiltrating but more competent effector cells. The biological logic is straightforward: immune infiltration is not synonymous with immune control.
Macrophage subsets diverged in orientation across subtypes
Macrophages added another layer of heterogeneity. The analysis identified macrophage subsets, including an NDUFB8+ population, with divergent functional orientations across the breast cancer subtypes. This matters because macrophages sit at the intersection of antigen presentation, tissue remodeling, inflammatory signaling, and immune suppression. Their phenotype can tilt the local tumor ecology toward immune activation or immune escape.
An NDUFB8+ designation points to a transcriptionally defined subgroup rather than a generic myeloid pool. In a tumor setting, such precision is clinically useful because macrophages are often the cells that decide whether T-cell traffic will be welcomed, redirected, or silenced. Subtype-specific macrophage behavior suggests that HER2-positive, TNBC, and ER-positive tumors may differ not only in how many macrophages they contain but in what those macrophages are doing inside the tissue. For therapeutic design, that could affect how investigators think about combining macrophage-directed interventions with immunotherapy.
B-cell remodeling in TNBC points to altered humoral immunity
B cells were also reshaped in TNBC. The study found a decreased proportion of plasmablasts and an increased proportion of memory B cells, alongside broader functional alterations. This pattern suggests that the humoral arm of immunity is not merely present or absent; it is reorganized into a different balance of activation and persistence.
Plasmablasts usually reflect active antibody-secreting responses, while memory B cells imply a more experienced, potentially durable immune compartment. A drop in plasmablast proportion alongside expansion of memory B cells may indicate that the TNBC microenvironment favors immune retention over active effector output. Whether that state is protective, exhausted, or repurposed by the tumor depends on the surrounding signals, and those signals are precisely what single-cell approaches are designed to capture.
Transcription factors and signaling networks offer subtype-specific clues
SCENIC analysis identified candidate transcription factors with subtype-specific patterns. E2F8 emerged across all subtypes, while ATF7 was linked to HER2-positive tumors. Their activity correlated with clinical outcomes, suggesting that immune behavior in breast cancer is not only a matter of cell composition but also of regulatory wiring inside those cells.
This is where the work moves from descriptive mapping to mechanistic hypothesis generation. Transcription factors govern cell fate decisions, activation thresholds, and survival programs. If a factor such as E2F8 is repeatedly observed across molecular subtypes, it may represent a shared regulatory node in the breast cancer immune ecosystem. ATF7, by contrast, appears more selectively tied to HER2-positive disease, raising the possibility that HER2-driven tumors use a distinct transcriptional circuit to shape their immune surroundings. No claim of clinical utility should be made from this analysis alone, but the regulatory signals are precise enough to justify follow-up studies.
Intercellular communication analysis also revealed both conserved and subtype-specific signaling pathways. That is clinically important because immune cells in tumors do not act independently; they exchange cytokines, ligands, and receptor-mediated instructions in a dense local network. When those conversations differ by subtype, immune evasion may also differ by subtype. A therapy that works by restoring one pathway may fail if another tumor subtype relies on a different set of signals to suppress immunity.
Why this atlas matters for subtype-specific immunotherapy
The study positions breast cancer immunology as a subtype-dependent system rather than a universal one. HER2-positive, TNBC, and ER-positive tumors may all share the diagnosis of breast cancer, yet their immune microenvironments are organized differently enough to require separate therapeutic thinking. Single-cell transcriptomics makes those differences visible at a resolution that conventional assays cannot match.
This is especially relevant for immunotherapy development, where response rates often reflect not just the treatment chosen but the immune readiness of the tumor itself. A tumor enriched for dysfunctional CD8+ T cells, altered macrophage subsets, and reshaped B-cell populations presents a different challenge from one with a more balanced immune ecosystem. The transcriptional atlas described here offers a framework for matching immune intervention to subtype-specific biology rather than treating all breast cancers as immunologically equivalent.
The work also leaves important questions open. The study is transcriptomic, not interventional, so it maps immune states without proving which of them drive outcome or treatment response. Functional validation will be needed before any of the candidate transcription factors or communication pathways can be translated into biomarkers or therapeutic targets. For now, the clearest value lies in definition: breast cancer subtypes carry distinct immune identities, and those identities are detailed enough to guide the next round of mechanistic research.