Differential Impact of Six JAK Inhibitors on Myeloid Cell Differentiation and Their T Cell Stimulatory Capacity: An In Vitro Comparative Study

Background Janus kinase inhibitors (JAKis) play a key role in treating autoimmune diseases; however, their immunomodulatory mechanisms, particularly regarding myeloid-derived suppressor cells (MDSCs) and dendritic cells (DCs), remain insufficiently understood. This study evaluated the effects of six approved JAKis; tofacitinib (TOF), baricitinib (BAR), peficitinib (PEF), upadacitinib (UPA), filgotinib (FIL), and deucravacitinib (DEU), on the differentiation and maturation of MDSCs and DCs, as well as their influence on T cell responses in vitro. Methods Bone marrow cells from SKG mice were cultured with GM-CSF and IL-4 to facilitate myeloid cell differentiation at varying JAKis concentrations. The proportion of MDSCs and DCs was measured using flow cytometry. DC maturation was evaluated by analyzing MHC class II expression following LPS stimulation. Cell viability was assessed using WST-8 and Annexin V assays. To investigate the effects on T cell responses, myeloid cells differentiated with JAKis were co-cultured with T cells and T cell proliferation was measured using CFSE dilution. Results JAKis differentially modulated myeloid cell differentiation. All six JAKis increased the proportion of MDSCs, with BAR and UPA inducing this expansion even at lower concentrations. TOF, BAR, PEF, and UPA suppressed DC differentiation, with BAR and UPA demonstrating efficacy at lower doses than the others. In contrast, FIL and DEU exhibited minimal effects on DC differentiation. None of the JAK inhibitors significantly altered MHC class II expression on MDSCs or DCs following LPS stimulation, indicating no effect on DC maturation. Notably, myeloid cells differentiated in the presence of BAR or UPA demonstrated markedly reduced T cell stimulatory capacity compared to DMSO-treated controls, which promoted robust T cell proliferation. Conversely, myeloid cells generated with TOF, PEF, FIL, or DEU retained comparable or only slightly reduced stimulatory effects, indicating that only BAR and UPA substantially attenuated the T cell-activating potential of the myeloid compartment. Conclusions JAK inhibitors have distinct impacts on myeloid cell lineages, promoting MDSC proliferation and variably suppressing DC differentiation. BAR and UPA effectively reduce the T cell stimulatory function of myeloid cells. These results demonstrate the importance of JAK selectivity in determining myeloid cell composition and modulating adaptive immune responses.