Circulating lymphocyte trafficking to the bone marrow contributes to lymphopenia in myocardial infarction

Some patients with myocardial infarction (MI) exhibit lymphopenia, a reduction in blood lymphocyte count. Moreover, lymphopenia inversely correlates with patient prognosis. The objective of this study was to elucidate the underlying mechanisms that cause lymphopenia after MI. Multiparameter flow cytometric analysis demonstrated that MI induced profound B and T lymphopenia in a mouse model, peaking at day 1 post-MI. The finding that non-MI control and MI mice exhibited similar apoptotic rate for blood B and T lymphocytes argues against apoptosis being essential for MI-induced lymphopenia. Interestingly, the bone marrow in day 1 post-MI mice contained more B and T cells but showed less B- and T-cell proliferation compared with day 0 controls. This suggests that blood lymphocytes may travel to the bone marrow after MI. This was confirmed by adoptive transfer experiments demonstrating that MI caused the loss of transferred lymphocytes in the blood, but the accumulation of transferred lymphocytes in the bone marrow. To elucidate the underlying signaling pathways, β₂-adrenergic receptor or sphingosine-1-phosphate receptor type 1 (S1PR1) was pharmacologically blocked, respectively. β₂-receptor inhibition had no significant effect on blood lymphocyte count, whereas S1PR1 blockade aggravated lymphopenia in MI mice. Furthermore, we discovered that MI-induced glucocorticoid release triggered lymphopenia. This was supported by the findings that adrenalectomy (ADX) completely prevented mice from MI-induced lymphopenia, and supplementation with corticosterone in adrenalectomized MI mice reinduced lymphopenia. In conclusion, our study demonstrates that MI-associated lymphopenia involves lymphocyte redistribution from peripheral blood to the bone marrow, which is mediated by glucocorticoids.NEW & NOTEWORTHY Lymphopenia, a reduction in blood lymphocyte count, is known to inversely correlate with the prognosis for patients with myocardial infarction (MI). However, the underlying mechanisms by which cardiac ischemia induces lymphopenia remain elusive. This study provides the first evidence that MI activates the hypothalamic-pituitary-adrenal (HPA) axis to increase glucocorticoid secretion, and elevated circulating glucocorticoids induce blood lymphocytes trafficking to the bone marrow, leading to lymphopenia.