Bhle40⁺ CD4⁺ donor T cells secrete GM-CSF that enhances indirect alloantigen presentation by dendritic cells amplifying GVHD in allogeneic hematopoietic stem cell transplant recipients.  The sequence of events in the pathway proposed by Piper et al is shown with local production of GM-CSF (2)  by donor T cells in the interstitium of the gut recruiting monocytes (3) that are matured into conventional dendritic cells (4), leading to activation of donor T cells primed to allo-antigens that cause apoptosis of gut epithelial cells (5), leakage of gut microbes into interstitium (6), activation of macrophages that phagocytose bacteria (7), release of inflammatory cytokines (8), and amplification of GVHD by activation of conventional donor T cells (9). DC, dendritic cells; TLR, Toll-like receptor.