Time Course of the Immune Response in Three Different Experimental Myocardial Infarction Models

Objectives: During myocardial infarction (MI), an immune response takes place to remove debris and form a stable scar. Experimentally, three different MI models are standardly used, the cryolesion (cryo) and the permanent (LAD) or intermittent (I/R) occlusion of the left anterior descending coronary artery. Due to the different pathomechanisms of tissue damage (necrosis vs. hypoxia/apoptosis), we investigated time course as well as quantitative and qualitative differences in the immunological response.

Methods: MI was induced in 10- to 12-week-old C57/Bl6J mice by cryo, LAD, or I/R. In sham operated mice, a thoracotomy was performed. The immunological response was investigated at postinfarction day (d) 1, d3, d7, and d14 (n = 3–7 each). To this aim hearts were excised, dissociated, and stained for FACS or fixed and sectioned for histological analysis. Heart function was measured by echocardiography (echo) on d7, d14, or d56.

Results: FACS analysis revealed increased leukocyte infiltration into the infarction area in I/R compared to cryo and LAD on d1 (CD45+/total cells on d1: I/R 19.5%, cryo 10.2%, LAD 8.8%, sham 7.1%). In cryo and LAD, leukocyte infiltration peaked on d3 (cryo 42.7%, LAD 30.7%), while in I/R there was no further increase. On d7, leukocyte amounts dropped in all three MI models (I/R 14.9%, cryo 12.6%, LAD 11.6%). In I/R also, monocytes were already present on d1, especially proinflammatory Ly6C hi and CCR2+ cells (Ly6C hi 15.0%, CCR2+ 6.9% of total cells), while this was delayed to d3 in cryo and LAD (Ly6C hi 16.3%, 11.6%; CCR2+ 6.9%, 4.4%, respectively). Macrophages peaked only in cryo on d3 (16.5% of total cells), in LAD these cells remained unchanged between d1 and d3 (6.2%, 6.6%) at lower levels. Interestingly, in I/R no macrophages were detected on d1, and only low levels were found at d3 and d7 (5.4%, 3.4%). Histology confirmed FACS data. Significant amounts of CD45+ cells were present in the infarction area on d3 postinfarction in all models (CD45+ cells/mm²: cryo 1,138; LAD 1,485; I/R 1,499; sham 44) and decreased markedly already on d7. Echo showed reduced fractional shortening (FS) in cryo and LAD already on d7 (sham 36.8%, cryo, LAD 24.4%). At d14, all models exhibited equally reduced FS, which remained stable until d56 (ca. 21%).

Conclusion: The immunological time course reveals temporal and quantitative differences in cryo, LAD and I/R. Proinflammatory monocytes appear earlier in I/R lesions compared to cryo or LAD, which might have consequences for lesion size and scar formation.