MHC class II molecules
In 1967, a puzzling observation was made by Calne and his laboratory team, during liver transplantation experiments on pigs. One animal survived for months, after receiving an allograft, without any immunosupression. Later, it was found that the soluble factors, which were secreted by the liver allograft and were responsible for the development of tolerance to the recipient animal, were soluble MHC (sMHC) molecules.From this point, many laboratories around the world entered the quest, for the elucidation of the
immunoregulatory effects of the various soluble forms of MHC molecules. Today, it is widely accepted that body fluids, isolated from healthy individuals contain various amounts of soluble MHC molecules. The origin of these molecules has not been clearly determined and it could be the result of shedding, alternative splicing or active secretion, whereas it has been demonstrated that trophoblast cells secrete to the medium soluble HLA-DR molecules when stimulated by IFNγ. In the 70s and 80s during the study of T suppressors (some of which might correspond to the modern T regulatory cells), it was suggested that they affected nearby cells secreting various factors with MHC determinants.
Deviations from the concentration of these molecules in the body fluids of healthy individuals, have been recorder in numerous pathological conditions, such as viral encephalitis, rheumatoid arthritis, pathologic pregnancies, asthma and AIDS. These results imply that soluble MHC molecules have a special role, in the pathology of these diseases but it is not clarified whether they contribute to the pathology or if they are by-products of the pathological condition (they could have a potential role in the diagnosis of the disease stage). Additional research has confirmed that soluble forms of MHC molecules can have various immunomodulating effects, such as induction of apoptosis of CD4+ activation of CD8+ T cells and down-regulation of NK activity. The aim of this study was to perform a downstream investigation of the possible immunomodulating action of soluble MHC molecules isolated from the serum of male and pregnant Balb/c mice. We selected the 12th day of gestation for the isolation of the serum, according to Ranella A. (The role and the regulatory mechanisms of growth factors, membrane and secreted MHC class II molecules in the survival of the hemiallogeneic embryo during gestation, Ph.D. thesis, University of Crete, unpublished data), because the titer of the serum reaches a maximum. During the study we used affinity chromatography and immunoprecipitation for the isolation of sMHC II from the serum and cell culture techniques, in order to reveal any effect on freshly isolated spleen cells. Pregnancy is often called immune paradox, because the immune system of the mother allows to the fetus to survive although it is an allotransplant. Generally, pregnancy is largely considered to be a TH2 phenomenon that is characterized by downregulation of the immune system in the point where the mothers body meets the fetus. Soluble MHC class II molecules secreted by trophoblast cells could contribute to the development of tolerance, because the ability of antigen presentation from the trophoblast is lost along with the secreted MHC II. However, it has not been determined whether the molecules we isolate from the serum originate from the trophoblast. The presence of the same molecules in the serum of male mice suggests an alternative source. Our results revealed the following information:
- SDS-PAGE assays revealed a prominent band at about 67kDa.<br\>
- Upon the addition of these molecules, spleen cells from syngeneic mice respond positively and upregulate their activation status.<br\>
- In Mixed Lymphocyte Reactions, which depend on the successful activation of CD4+ T helpers, the incorporation of radioactivity was boosted when sMHC II (from pregnant) were added to Balb/c spleen cells, which responded to inactivated with mitomycin C C3H/HeN spleen cells, CD4 receptor seems to be important for the effect we recorded, since the incorporation of radioactivity diminishes when we interfere with it.<br\>
- An important decrease in the level of IgM has also been recorded when sMHC from pregnant and male mice were added to the medium. This could result from interference of cell-cell interactions or by favoring the expression of Type-1 cytokine profile.<br\>
- Type-1 cytokines were increased in the samples that were treated with sMHC II from male mice.<br\>
- Type-2 cytokines were increased in the samples that were treated with sMHC II from pregnant mice.<br\>
- Our soluble molecules have the potential to cause robust activation of spleen cells, which is comparable with the activation caused by up to 100000cfu of Legionella pneumophila ser.1<br\>
- When spleen cells were challenged with L.pneumophila fragments the addition of sMHC II from male mice produced a Type-1 cytokine profile, whereas the addition of sMHC II from female mice a Type-2 profile (data notpresented). However, both molecules seemed to upregulate the production of IL-15.<br\>
- Soluble molecules isolated from female pregnant Balb/c mice might have the potential to downregulate the proliferation of syngeneic placenta cells (preliminary data, not presented).
