Caveolae and Caveolin in Immune Cells: Distribution and Functions

Purpose: Caveolae and caveolin are cholesterol-rich membrane microdomains involved in endocytosis, cholesterol regulation, and signal transduction, but their presence and function in immune cells have been contentious. This review addresses the need to clarify the distribution and roles of caveolae/caveolin in mammalian immune cells, especially given emerging evidence that they mediate pathogen internalization by antigen-presenting cells and participate in immune-cell signaling. ---

Hypothesis: The review’s central thesis is: if caveolae and caveolin are expressed in immune cells—particularly myeloid cells and possibly lymphoid cells depending on activation/maturation state—then they contribute to immune-cell functions including pathogen uptake, cholesterol transport, and signal transduction, and their expression is cell-type-, species-, and activation-dependent. ---

Aims: - Review the distribution of caveolae and caveolin across different immune cell types. - Summarize interactions between caveolae and pathogens, including bacterial, viral, and toxin entry. - Discuss other functions of caveolae/caveolin relevant to immune cells: cholesterol transport/regulation and signal transduction. - Highlight controversies, technical limitations, and future research directions. ---

Delivery system: This is not an engineered delivery system review. It focuses on endogenous caveolae as membrane platforms: - Caveolae: flask-shaped, cholesterol- and glycosphingolipid-rich, caveolin-positive membrane invaginations (~50–100 nm); can also be flat, tubular, detached vesicles, or fuse into larger structures. - Caveolin isoforms: caveolin-1 (α and β), caveolin-2, and caveolin-3 (muscle-specific). Caveolin-1 and -2 form hetero-oligomeric complexes. - Pathogen entry routes: FimH-expressing E. coli via GPI-linked CD48; Mycobacterium spp.; Chlamydia trachomatis; cholera toxin via GM1; Campylobacter jejuni; simian virus 40 (SV40) via MHC class I; respiratory syncytial virus (RSV) in bovine dendritic cells; HIV via lipid rafts. - Associated surface molecules (Table 1): BCR, CD19/CD21, TCR, IL-2Rα, HLA class I/II, CD48, IFN-γR, CR3, SR-BI/SR-BII, TNFR1, CD4, CD45, CD55, CD14, CD59. ---

Approach: Narrative review of published literature. No experimental groups. Model systems discussed include: - Primary immune cells: murine macrophages and mast cells; human dendritic cells and neutrophils; bovine monocytes, macrophages, and dendritic cells; human peripheral blood lymphocytes; rat resident and elicited peritoneal macrophages. - Cell lines: Jurkat T cells, U937 promonocytes, HEp-2, HeLa 299, J-774A.1, Caco-2, CV-1. - Pathogens/toxins: E. coli, Mycobacterium bovis BCG, Mycobacterium kansasii, Chlamydia trachomatis, cholera toxin, Campylobacter jejuni, SV40, RSV, HIV-1. - Disease context: Host–pathogen interactions, antigen presentation, immune cell signaling, cholesterol homeostasis. ---

Key methods: Techniques highlighted across cited studies: - Confocal microscopy for caveolin-1 staining and colocalization. - Electron microscopy for caveolae ultrastructure. - Western blotting for caveolin expression. - Cholesterol depletion agents: filipin, nystatin, methyl-β-cyclodextrin. - GPI-anchor removal with phosphatidylinositol phospholipase C (PI-PLC). - Live fluorescence microscopy for pathogen internalization and trafficking. - Antibody staining and colocalization analysis. - Functional inhibition assays for pathogen uptake and T cell presentation. ---

Key results: 1. Distribution is contentious and context-dependent: Caveolae/caveolin are commonly found in myeloid cells (macrophages, mast cells, dendritic cells, neutrophils) but not consistently in lymphoid cells. More recent evidence suggests they may be present in all immune cell types, with expression/distribution dependent on activation and/or maturation state. 2. Species and cell-type differences in caveolin localization: Human CD26+ and CD21+ peripheral blood lymphocytes stain for caveolin-1 at or near the cell surface; bovine lymphocytes show predominantly perinuclear staining. Human dendritic cells stain at the cell surface, whereas bovine dendritic cells stain perinuclearly. Rat resident peritoneal macrophages have sparse caveolae, but complete Freund’s adjuvant increases caveolae and caveolin-1 at the plasma membrane. Mast cells have vesicular and plasmalemmal caveolae rather than typical flask-shaped invaginations. 3. Caveolae mediate pathogen entry and immune evasion: FimH+ E. coli binds CD48 in macrophage/mast cell caveolae; Chlamydia trachomatis enters via caveolin-positive, cholesterol-rich domains and replicates in caveolin-positive vesicles that avoid lysosomes; cholera toxin binds GM1 in caveolae and traffics to Golgi/ER; SV40 enters via MHC class I and traffics through caveosomes to the ER; RSV colocalizes with caveolin in bovine dendritic cells, and filipin inhibits viral antigen presentation to CD4+ T cells. 4. Caveolin functions in cholesterol transport and signaling: Caveolin binds free cholesterol and cycles between the plasma membrane, ERGIC, and Golgi. Caveolae concentrate signaling molecules such as Src-family kinases, G proteins, H-ras, eNOS, and hormone receptors. Cholesterol oxidation displaces caveolin from the plasma membrane to intracellular vesicles. ---

Interpretation: The authors conclude that caveolae represent an important mechanism in host–pathogen interactions and immune-cell function. By targeting caveolae, pathogens can traffic to the Golgi and/or ER and avoid lysosomal degradation. Understanding these mechanisms may aid development of new strategies to combat infection. However, the role of caveolae in immune-cell function, although expanding rapidly, is still in its infancy. ---

Limitations: - No consensus on the presence of caveolae/caveolin in immune cells; findings are cell-type-, species-, and activation-dependent. - Many studies rely on pharmacological inhibitors (filipin, nystatin, methyl-β-cyclodextrin) with uncertain specificity; methyl-β-cyclodextrin can also inhibit clathrin-coated pit formation. - Antibody specificity issues: some anti-caveolin antibodies cross-react with BCG and Schistosoma mansoni. - Distinction between caveolae and lipid rafts is often blurred. - Many findings come from cell lines rather than primary immune cells. - Direct evidence for caveolae/caveolin involvement in immune-cell signal transduction is still lacking. - Species differences (human vs bovine) complicate generalization.