Immunohistochemical Assessment of Cardiac Macrophages in the Aged Fischer 344 Rat

Macrophages have multiple roles in the heart including immune surveillance and extracellular matrix remodeling. Aging increases both collagen deposition and macrophage number in the heart; however, rodent models used to study cardiac macrophages have age-related comorbidities such as atherosclerosis and hypertension. The Fischer 344 rat does not develop these conditions with aging; therefore, the purpose of this study was to evaluate macrophage number and polarization in the hearts of aged (24-month) and young (6-month) Fischer 344 rats. Paraffin-embedded hearts were assessed for collagen deposition and immunolabeled for CD68, CD163, CD206, and galectin-3. Compared with young rats, significantly greater collagen deposition was observed in the old rats. There were no significant differences in CD68⁺ or CD163⁺ cells between age groups, but both CD206⁺ and galectin-3⁺ cells were more numerous in the aged animals. Double-immunofluorescence studies demonstrated that galectin-3 colocalized with both CD68 and CD163, suggesting that galectin-3 is found in cardiac macrophages. Further colocalization studies demonstrated similar proportions of CD68⁺/CD163^-, CD68⁺/CD163⁺, and CD68^-/CD163⁺ cells between age groups, suggesting that aging does not affect macrophage polarization. As CD206⁺ and galectin-3⁺ cells promote fibrosis, these results warrant future studies that delineate the specific roles of these cells in the aged heart.