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Abstract: Inflammatory reactions driven by an accumulation in the intima of immune-inflammatory cells and focal lipid depositions are the hallmarks of atherogenesis. It is commonly accepted that immune-inflammatory cell accumulation and lipid deposition are associated with the very earlier stage of atherosclerosis but no study has yet focused on the determination of quantitative values of this association. The present study examined correlations between lipid deposition, immune-inflammatory cell content and major histocompatibility complex (MHC) class II molecule HLA-DR expression in diffuse intimal thickening (DIT), which is thought to represent the earliest macroscopic manifestation of atherosclerosis. In parallel consecutive tissue sections of DIT, lipids were examined by chromatographic analysis (including triglycerides, cholesteryl esters, free cholesterol and phospholipids), histochemically, using Oil Red O staining, and by electron microscopy. Immune-inflammatory cells and HLA-DR expression were examined immunohistochemically in consecutive sections of the same tissue specimens. The study revealed that lipids exhibited a non-uniform distribution throughout the intima. In the juxtaluminal sublayer, lipids were localized both intracellularly and extracellularly, whereas in the juxtamedial musculoelastic sublayer, lipids were present predominantly along elastic fibers. Lipid deposits were found to positively correlate with HLA-DR expression (r =0.79; p <0.001). The study also identified a positive correlation between lipid deposition and immune-inflammatory cell content but the correlation values varied between different sublayers of the tunica intima. The correlation between lipid deposition and immune-inflammatory cell content in the juxtaluminal sublayer of the intima was notably stronger (r =0.69; p <0.001) than in the juxtamedial musculoelastic layer (r =0.28; p <0.001). The findings of the present study support a view that lipid accumulation in the intima plays a role in the initiation of inflammatory reaction and that at the pre-lesional stage in the development of atherosclerosis, lipid-associated immune cell activation might occur primarily in the juxtaluminal portion of the intima. [Copyright &y& Elsevier]