Human leukocyte antigens (HLAs) are encoded by the major histocompatibility gene complex (MHC) on chromosome 6p and function as major histocompatibility antigens in transplantation. The natural function of HLA is to present antigenic peptides to specific T cells. Exogenous and endogenous proteins present within human cells are continually broken down into peptides. HLA molecules on professional APCs bind those peptides within their groove and present them from the cell surface to the T-cell receptors on T lymphocytes. Inasmuch as the human genome includes greater than 10⁷ polymorphic sequences, there is great likelihood that donor and recipient differ for one or more of the proteins that are presented as HLA:peptide complexes to T cells. These polymorphic proteins function as minor histocompatibility antigens (mHAs) in transplantation. Autoreactive T-cell recognition of “self” peptides in the thymus leads to negative selection (death), so that the probability of an individual developing autoimmunity is minimized. After transplantation, this delicate balance of self and nonself is perturbed. Donor and recipient may differ for HLA molecules or mHA. Each peripheral T cell harbors a unique T-cell antigen receptor; therefore, the human repertoire for recognition of foreign antigens is enormous. Donor and recipient HLA mismatching plays a critical role in the development of GVHD, because each type of HLA presents a unique repertoire of antigenic peptides. When recipient cells are mismatched at an HLA locus, the donor T cells have not undergone thymic deletion to avoid recognizing those HLA:peptide complexes. Therefore, the degree of HLA disparity directly increases the probability and severity of acute GVHD following transplantation.⁴, ⁵ For umbilical cord blood donors a greater degree of HLA mismatch is tolerated presumably due to the small number of T cells within the cord blood (˜10⁴ per kg recipient weight) as compared to adult donor blood (˜10⁸) or marrow (˜10⁷).

mHA are polymorphic proteins genetically encoded anywhere outside HLA. Host mHA proteins are presented as HLA:peptide complexes to donor T cells and elicit an immune response even when presented via a matched HLA molecule. mHA mismatches are only partially characterized for the ability to affect acute GVHD. Some mHA have broad tissue expression such as the male-associated H-Y antigen.⁶ Alternatively, HA-1 and HA-2, for example, have their expression restricted to hematopoietic cells, including leukemia, and immune cells.⁷ It is this variability of tissue mHA expression that may allow for tailoring therapy toward hematopoietic-restricted mHA to promote GVT response without GVHD. Efforts to prevent GVHD would have to target immune responses against the ubiquitously expressed mHA.⁶, ⁸, ⁹

Other triggers of GVHD include molecules that activate the innate immune system. Pre-transplant radiation or chemotherapy damages host cells and enteric microbes that contain damage-associated molecular patterns (DAMPs) or pathogen-associated molecular patterns (PAMPs), respectively. DAMPs such as proteases
or ions released from damaged epithelial cells and PAMPs such as lipopolysaccharide within common bacteria, can activate Toll-like receptors setting off a cascade of immune events characterized by proinflammatory cytokine secretion.¹⁰ To a large degree these processes are responsible for initiation of the “cytokine storm” described as critical in earlier models of acute GVHD.¹¹

Sensors of GVHD refer to the cells and processes that recognize the mHA or HLA mismatches as foreign, thereby setting the immune response in motion. Antigens are presented by host professional APC such as dendritic cells, macrophages, or Langerhans cells. Host dendritic cells may be primed for antigen presentation via the cytokine storm accompanying the conditioning therapy. The nature of antigen presentation interactions includes direct presentation, which mimics physiologic APC-T-cell interactions. Specifically, direct presentation consists of host APC presenting endogenous (cytosolic) host peptide to donor CD8 T cells, and exogenous (extracellular, endocytosed peptide, processed in lysosomes) antigens are presented to donor CD4 T-cells. Indirect presentation specifies exogenous antigen presented from donor APC to donor CD4 cells. Cross-presentation refers to exogenous antigen (derived from host cells) presented to donor T cells from donor APC. Over time APC change from primarily host origin to donor origin. It is likely that direct presentation by host APC is predominant during early stages of acute GVHD, whereas indirect or cross-presentation by donor APC is predominant in chronic GVHD. APC-T-cell interaction involves not only interaction between HLA plus cognate antigen with T cell receptor (TCR), but co-stimulatory and inhibitory signals at the immune synapse, secondary signals from cell subsets, and paracrine or autocrine mediators. APC provide necessary co-stimulatory signals directly when activating T cells, such as CD28, CD40, and ICOS as well as inhibitory signals such as CTLA-4 and PD-1. Manipulation of these signals via antibody interference or small molecule blockade of downstream signaling events remains a field ripe for modulation of GVHD. Exogenous cell types critical to inhibition of APC include regulatory T cells, gamma-delta T cells in the GI tract, natural killer (NK) cells, and host NKT cells. T-cell response is an adaptive immune response which is often triggered by the innate immune response. As described, Toll-like receptors are sensors of the DAMP and PAMP release precipitated by the condition regimen. These Toll-like receptor interactions lead to release of proinflammatory cytokines such as tumor necrosis factor alpha (TNFα) and interleukin (IL)-6 which contribute to the inflammatory milieu surrounding T-cell-mediated GVHD damage.

GVHD is primarily mediated by donor T cells, and differing subsets that have normal physiologic activities outside the framework of transplant, act at different capacities to induce or inhibit GVHD development. Most animal models point to the relevance of naive T cells, rather than memory T cells, in the induction of GVHD. The process of homeostatic proliferation in the lymphoablated host causes brisk clonal expansion of the transferred donor T cells and plays an important role for the initiation of acute GVHD. Donor regulatory T cells (Treg) repress the GVHD responses. Treg are CD4 T cells that express the FOXP3 transcription factor, high levels of the alpha chain of the IL-2 receptor, but no IL-7 receptor and function as a natural suppressor to maintain peripheral tolerance. These donor-derived regulatory T cells inhibit the activation and proliferation of donor T cells implicated in the pathogenesis of acute GVHD and may spare the GVT effect.¹² Recent data show that low-dose IL-2 expands regulatory T cells in vivo and may be effective therapy for established GVHD,¹³ whereas anti-IL-2 receptor antibodies, tested before Treg importance and biology were recognized, appear to accelerate GVHD.¹⁴

Additional CD4 T-cell subsets are critical. It is Th1 cells, characterized by IL-2 and interferon gamma (IFNγ) secretion, which
are the likely main T-cell mediators of acute GVHD. Th2 T cells have been linked to protection against acute GVHD. Th17 CD4 cells secrete IL-17 and function distinctly from Th1 cells in a manner that is not yet fully characterized in the context of human GVHD. Murine transplant models provide evidence that Th17 can cause GVHD.¹⁵, ¹⁶ T-cell trafficking is another important step in GVHD pathogenesis. Differential expression of chemokines on GVHD target organs and draining lymph nodes mediate differential trafficking of T-cell subsets expressing different chemokine receptors. An initial report indicated that a CCR5 inhibitor can prevent GVHD in gut and liver.¹⁷

The effectors of acute GVHD are the cytolytic cellular elements and inflammatory cytokines. Activated CD4 and CD8 cytotoxic T lymphocytes are the primary effectors of GVHD. Upon interaction with their cognate HLA:peptide they act to lyse or cause apoptosis of target cells via secretion of perforin and granzyme or binding of FAS, respectively. IFNγ and TNFα are both critical to the inflammatory process of GVHD. Inflammatory cytokines induce and amplify cellular damage.