The link between major histocompatibility complex antibodies and cell proliferation

Abstract

Experimental evidence indicates that donor-specific antibodies targeting major histocompatibility complex classes I and II molecules can elicit the key features of transplant vasculopathy by acting on the graft vasculature in 3 ways: directly activating proliferative, prosurvival, and migratory signaling in the target endothelial and smooth muscle cells; increasing expression of mitogenic factors in vascular endothelial cells, creating a potential proliferative autocrine loop; and promoting recruitment of inflammatory cells that produce mitogenic factors and elicit chronic inflammation, proliferation, and fibrosis. Here, we review the experimental literature showing the complement and Fc-independent effects of major histocompatibility complex classes I and II antibodies on graft vascular cells that may directly contribute to the proliferative aspect of transplant vasculopathy.

Figure 1

Crosslinking of HLA class I molecules by antibodies induces cell signaling pathways which promote cell growth. FAK activates paxillin, an important regulator of the cytoskeleton. FAK activation also permits complex formation with Src. PI3K-dependent activation of Akt through PDK1 is central to cell growth. Akt inhibits GSK3beta, an antagonizer of cyclins such as cyclin E, which is vital to cell cycle progression. Akt also stimulates cyclin-dependent kinase 2, which causes the tumor suppressor Rb to release the transcription factor E2F, facilitating production of genes which promote G1/S transition. Akt targets mTOR. mTOR complex 1 is responsible for S6K phosphorylation, which in turn activates S6 ribosomal protein. 4EBP-1, an inhibitor of the translation factor eIF-4E, is phosphorylated in an mTOR dependent manner after HLA class I molecule crosslinking. Phosphorylation ultimately triggers release of eIF-4E and allows recruitment of 40S ribosomal subunits to mRNA. Cumulatively, these events result in increased protein synthesis and are central to cell proliferation. Finally, crosslinking of HLA class I molecules results in rapid translocation of FGFR to the cell surface and increased sensitivity to growth factor. The ERK MAP kinase is phosphorylated in an mTORC2 and FGF dependent manner, and actively promotes proliferation.

Figure 2

Crosslinking of HLA class I molecules with antibodies activates Akt, which promotes cell survival. PI3K catalyzes phosphorylation of PIP₂ to yield PIP₃ which can interact with a discrete region known as the pleckstrin homology domain. Akt and PDK1 localize to the membrane in response to PIP3, where PDK1 phosphorylates Akt at Thr308. Akt promotes mTORC1 activation, while mTORC2 phosphorylates Akt at Ser473. Akt increases Bcl-2 and Bcl-xL levels in the cell and phosphorylates Bad, a pro-apoptotic mediator, which allows 14-3-3 proteins to bind. This association with 14-3-3 prevents Bad antagonism of Bcl-2 and Bcl-xL proteins and promotes cell survival. Finally, cells treated with HLA class I antibody upregulate the antioxidant HO-1 in a PI3K dependent manner, which contributes to the cells' resistance to complement mediated lysis.

Figure 3

Three-step model of the Fc independent effects of HLA class I antibodies on vascular endothelium. HLA class I molecule ligation immediately activates cell signaling pathways which cause cell cycle progression, promote cell survival and protein synthesis, and ultimately induce cellular proliferation. HLA class I molecule crosslinking also activates transcription, and endothelial cells begin producing inflammatory and proliferative factors such as IL-8, VEGF and MCP-1 after a lag period. Finally, HLA class I molecule signaling mobilizes endothelial vesicles known as Weibel-Palade bodies (WPb), resulting in presentation of the adhesion molecule P-selectin on the cell surface and increasing the binding of leukocytes. Immune cells such as T cells and macrophages can release mediators in the neointima which cause endothelial and smooth muscle cell proliferation and vascular remodeling.