During inflammation, i.e., in the context of cancer and infection, metabolites serve as key mediators of immunoregulatory processes. For instance, adenosine, itaconate, and lactate were shown to modulate immune cell recruitment and activity. In a preliminary study, kidney tissue from pyelonephritis and control patients was analyzed by label-free LC-MS/MS on a quadrupole-ion trap-orbitrap MS (Orbitrap Fusion, Thermo Fisher) coupled to a nano-UPLC (Dionex Ultimate 3000 UPLC system, Thermo Fisher). Here, metabolic factors, such as lactate dehydrogenase A, were found to be positively correlated with CD163, a marker of macrophage polarization. This potential co-regulation of metabolic and immune processes might be facilitated through lactylation, a post-translational modification affecting protein activity and gene expression patterns. To further study the polarizing effect of lactate, we employed a model of murine bone marrow-derived macrophages (BMDMs). Here, exposure to titrated concentrations of sodium-DL-lactate resulted in elevated levels of lactylation and CD163 expression, as assessed by flow cytometry. Additional LC-MS/MS-based proteomic analysis will allow in-depth characterization of both lactate-dependent macrophage polarization and the cell-specific lactylome. Future measurements will be performed on a timsTOF-HT (Bruker Daltonics) coupled to a nano-Elute-2 in DIA mode. Moreover, results of in vitro studies, i.e. regarding macrophage lactylation, will be validated in tissue from pyelonephritis and melanoma patients, further elucidating the interrelationship between metabolism and immunity in vivo. These findings will improve our understanding of the molecular factors that shape macrophage differentiation in inflammatory disorders.