Stem cell mobilization with G-CSF attenuates acute GVHD through effects on T cells and APCs. After SCT, tissue injury and local inflammation (including IL-1, IL-6, and TNF-α release) are initiated by the conditioning regimen and promote the activation of host APCs. The interaction between activated host APCs (displaying disparate histocompatibility antigens) and naive donor CD4⁺ and CD8⁺ T cells preferentially drives type 1 differentiation, generating large amounts of IFN-γ that primes mononuclear phagocytes of donor and host origin. Donor CD4⁺ T cell responses are subsequently perpetuated by donor APCs presenting host antigens. After activation by LPS and other gut-derived immunostimulants, monocytes primed by T_H1 cytokines secrete cytopathic quantities of proinflammatory cytokines (TNF-α, IL-1) and mediate tissue injury in the inflammatory effector pathway of GVHD. Concurrently, effector donor CD8⁺ T cells are expanded, gain cytolytic function, and mediate target tissue GVHD through their cellular cytolytic machinery (eg, perforin, granzyme, TRAIL) in the cytolytic effector pathway. This leads to the “cytokine storm” characteristic of acute GVHD, whereby target tissues are damaged in MHC-independent and -dependent fashion. After G-CSF mobilization of stem cell donors, however, 3 key immunomodulatory effects before transplantation lead to the attenuation of GVHD. First, donor T cells up-regulate GATA-3 expression and are biased toward T_H2 differentiation, limiting T_H1-dependent monocyte activation after SCT. Second, G-CSF induces the generation of Tr1 regulatory cells (distinct from classical CD4⁺CD25⁺ T_reg) through IL-10 production. Third, G-CSF expands regulatory APCs within the donor (immature myeloid precursors and plasmacytoid DCs) which, after transplantation, promote the generation of classical CD4⁺CD25⁺ IL-10–producing T_regs. The generation of IL-10 and TGF-β from Tr1 and T_reg serve to further inhibit the inflammatory effector phase of GVHD, limiting target tissue damage.