Potential role of soluble human leukocyte antigen-G molecules in multiple sclerosis

Nonclassical human leukocyte antigen (HLA)-G antigens in soluble form (sHLA-G) have recently been suggested to have a potential role as immunomodulatory factors in multiple sclerosis (MS), a chronic inflammatory demyelinating and neurodegenerative disease of the central nervous system of unknown etiology and supposed autoimmune origin. In MS patients, sHLA-G levels were elevated in cerebrospinal fluid (CSF), intrathecally synthesized, predominantly represented by the HLA-G5 isoform and even more elevated in cases of inactive disease, as determined by magnetic resonance imaging. In MS, CSF sHLA-G concentrations were also related to the formation of an intrathecal anti-inflammatory microenvironment based on a positive correlation to CSF interleukin-10 titers and an inverse association to the levels of antiapoptotic sFas molecules in the CSF. Expression of HLA-G antigens was detected in microglia, macrophages, and endothelial cells within and around MS lesions and was enhanced in microglial cells by T-helper-1 proinflammatory cytokines. A novel subpopulation of naturally occurring CD4+ and CD8+ regulatory T cells expressing HLA-G1 and secreting HLA-G5 was identified in the CSF of MS patients. Taken together, these findings seem to indicate that sHLA-G antigens may be implicated in the termination of MS autoimmunity and associated with remission of the disease through their function as anti-inflammatory molecules. However, the mechanisms operating in the immunomodulatory circuit mediated by sHLA-G proteins remain to be clarified.