The role of toll-like receptors in multifactorial mechanisms of early and late renal allotransplant injury, with a focus on the TLR4 receptor and mononuclear cells

The innate immune system is activated before an adaptive immune response. An expression of a particular toll-like receptor (TLR) in a transplanted kidney depends on the localization of specific cells (e.g., endothelium, elements of the nephron structure), recent pathology and the time passed since transplantation. The TLR4 receptor is expressed on renal tubular epithelial (RTE) and endothelial cells, podocytes, blood and interstitial monocytes/macrophages, and dendritic cells. While circulating in blood, some monocytes are attracted and penetrate the transplanted organ, where they supplement the donor's resident macrophages. The intensity of migration depends on the local activation of inflammation in the graft and on the expression of specific receptors on kidney endothelial cells and monocytes/macrophages. The percentage of cells with shifted TLR4 expression usually increases in circulating monocytes. The TLR4 and the biochemical stimulation cascade derived from it in any type of cell, including monocytes, undergo multi-level regulation with feedback loops with other components of the primary system, and are also dependent on the action of immunosuppression. Toll-like receptor 4 senses stimuli that make monocytes contribute differently both to acute/chronic kidney injuries and to the development of tolerance. After kidney transplantation, TLR4 expression and related cytokine production capacity may vary depending on past diseases and oncoming problems. Since conventional immunosuppression does not prevent chronic allograft injury (CAI), peripheral blood monocytes and TLR4 constitute candidates for diagnostic and therapeutic targets. Considering the mutual communication among various elements of the primary immune system, future therapeutic intervention should be directed toward factors directly or indirectly regulating the expression or post-receptor signaling of the TLR4 receptor.