Abstract
BackgroundPreviously we reported that Kala-azar patients show progressive decrease in serum cholesterol as a function of splenic parasite burden. Splenic macrophages (MΦ) of Leishmania donovani (LD) infected mice show decrease in membrane cholesterol, while LD infected macrophages (I-MΦ) show defective T cell stimulating ability that could be corrected by liposomal delivery of cholesterol. T helper cells recognize peptide antigen in the context of class II MHC molecule. It is known that the conformation of a large number of membrane proteins is dependent on membrane cholesterol. In this investigation we tried to understand the influence of decreased membrane cholesterol in I-MΦ on the conformation of MHC-II protein and peptide-MHC-II stability, and its bearing on the antigen specific T-cell activation.Methodology/Principal FindingsMΦ of CBA/j mice were infected with Leishmania donovani (I-MΦ). Two different anti-Aκ mAbs were used to monitor the status of MHC-II protein under parasitized condition. One of them (11.5–2) was conformation specific, whereas the other one (10.2.16) was not. Under parasitized condition, the binding of 11.5–2 decreased significantly with respect to the normal counterpart, whereas that of 10.2.16 remained unaltered. The binding of 11.5–2 was restored to normal upon liposomal delivery of cholesterol in I-MΦ. By molecular dynamics (MD) simulation studies we found that there was considerable conformational fluctuation in the transmembrane domain of the MHC-II protein in the presence of membrane cholesterol than in its absence, which possibly influenced the distal peptide binding groove. This was evident from the faster dissociation of the cognate peptide from peptide-MHC complex under parasitized condition, which could be corrected by liposomal delivery of cholesterol in I-MΦ.ConclusionThe decrease in membrane cholesterol in I-MΦ may lead to altered conformation of MHC II, and this may contribute to a faster dissociation of the peptide. Furthermore, liposomal delivery of cholesterol in I-MΦ restored its normal antigen presenting function. This observation brings strength to our previous observation on host directed therapeutic application of liposomal cholesterol in experimental visceral leishmaniasis.
Highlights
The disease visceral leishmaniasis or kala azar is characterized by the depression in the cellular immune response and the cause of which is largely unknown [1, 2]
The decrease in membrane cholesterol in I-MΦ may lead to altered conformation of MHC II, and this may contribute to a faster dissociation of the peptide
The MF from CBA/j mice were infected with Leishmania donovani (LD) (I-MF) and treated with liposomal cholesterol (I-MF-CL) or liposomal cholesterol analogue (I-MF-AL) or liposomal DPPC (I-MFDPPC)
Summary
The disease visceral leishmaniasis or kala azar is characterized by the depression in the cellular immune response and the cause of which is largely unknown [1, 2]. The protozoan parasite, Leishmania donovani (LD), the causative agent of visceral leishmaniasis, replicates within the macrophage or dendritic cells of the mammalian hosts [3]. The parasites during their intracellular life cycle in the macrophages disrupt lipid rafts [4] and are unable to form synapse with antigen specific T-cells [5]. Due to absence of miR122 the overall cholesterol metabolism is decreased leading to generalized defects in cholesterol biosynthesis in L. donovani infected hosts. Splenic macrophages (MΦ) of Leishmania donovani (LD) infected mice show decrease in membrane cholesterol, while LD infected macrophages (I-MΦ) show defective T cell stimulating ability that could be corrected by liposomal delivery of cholesterol. In this investigation we tried to understand the influence of decreased membrane cholesterol in I-MΦ on the conformation of MHC-II protein and peptide-MHC-II stability, and its bearing on the antigen specific T-cell activation
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