Pathological mechanisms of aGVHD. (Top) Microbial products and cytokines from resident cells (eg, effector T cells, CD163⁺ monocyte-derived macrophages) are believed to induce GVHD. (Bottom) While complete response to first-line therapy usually induces an equilibrium of host/donor tissue-resident T cells and replacement of host macrophages by donor APCs in barrier tissues, steroid resistance is now shown to be related to the occurrence of CXCR3⁺ donor T cells, activated tissue-resident T cells, plasmablasts, and dendritic cells. APC, antigen-presenting cells; DC, dendritic cells; IgM, immunoglobulin M. Figure designed by Johanna Strobl and created with BioRender.com.