Mitochondrial transplantation redirects rather than suppresses the inflammatory response in primary human monocytes

Circulating monocytes are recruited into inflamed tissue, where local cues determine whether the macrophages they become drive inflammation or resolve it. Because that decision occurs after cells leave the blood, the circulating stage offers a window to alter monocyte programming, and monocyte inflammatory function is closely coupled to mitochondrial metabolism. Here we transplanted freshly isolated, respiration-competent mitochondria from HEK293T cells into primary human CD14⁺ monocytes and characterized the resulting mitochondria-transplanted monocytes (MTM) across surface phenotype, respiration, and inflammatory function. Isolated mitochondria were intact and respiration-competent, with a respiratory control ratio of 8.7 and a coupling efficiency of 78.9 %. Monocytes internalized them in proportion to the dose supplied; within 3 h, 88% of viable cells were positive for labeled mitochondria, with no loss of viability detected; isolated mitochondria did not activate NF-κB or IRF reporters in THP1-Dual cells. Transplantation reduced the frequency of the pro-inflammatory intermediate subset, increased CD14 by 2.11-fold and HLA-DR by 1.31-fold, and decreased CD163. MTM showed reduced ATP-linked respiration with maximal capacity preserved, and a trend toward a higher maximal-to-basal respiration ratio. Following stimulation with LPS, MTM released markedly less TNF-α (0.06-fold) and CXCL10/IP-10 (0.25-fold) and, in most donors, more IL-10 (3.33-fold), while the remaining analytes were largely unchanged. Mitochondrial transplantation therefore shifts primary human monocytes toward a phenotype with selectively reduced inflammatory mediator release and increased IL-10 in most donors. It identifies circulating monocytes as a tractable target for mitochondria-based modulation of inflammation.