TLDR
HDL exerts direct immunomodulatory effects on monocytes, macrophages, and T cells. In T2D, these functions are impaired: HDL loses its ability to suppress monocyte activation, reduce macrophage cytokine release, and modulate T-cell responses. SAA-enriched HDL can become actively pro-inflammatory.
Key Immune Effects of HDL
- Monocytes: HDL suppresses CD11b expression and reduces monocyte adhesion to endothelium. rHDL infusion in T2D patients reduces CD11b.
- Macrophages: Promotes M2 (anti-inflammatory) polarization; suppresses TLR-induced inflammation via ATF3 induction; inhibits TNFα and IL-1β release. In T2D, glycated apoA-I loses ability to suppress cytokine release.
- T cells: HDL modulates T-cell antigen presentation via cholesterol efflux; apoA-I inhibits contact-mediated monocyte activation by T cells; apoE-containing HDL promotes regulatory T-cell survival.
- Neutrophils: HDL reduces ROS production induced by contact with activated T cells.
Dysfunction in T2D
- SAA enrichment of HDL in T2D increases TNFα secretion from PBMCs
- Glycated apoA-I fails to suppress LPS-induced TNFα/IL-1β in macrophages (via impaired NF-κB inhibition)
- MPO-oxidized HDL impairs ABCA1-dependent efflux, promoting foam cell formation
- Reduced PON1 activity diminishes HDL’s ability to neutralize oxidized lipids
- Impaired cholesterol efflux from macrophages affects antigen presentation to T cells
Relevance to PBMC Studies
HDL immune effects directly intersect with the PBMC changes observed in T2D: HDL dysfunction may contribute to the monocyte inflammatory signature, altered T-cell activity, and disrupted cholesterol homeostasis reported in T2D PBMC studies (Fakhoury et al., Gu et al.).