T-cells recognize the MHC molecules and body's own peptides. When it doesn't, it alarms the immune system. But do T-cells express MHC molecules ? If so, how are they using it? If not, what happens when a virus infects T-cells? (Yes, I am confused about HIV infection mechanism too. They escape from immune response by altering their genes and disrupting the MHC-peptide bonding. I can see that this can work in macrophages. But what is the situation in T-cells ?)
yes, T cells and all other nucleated express MHC class 1 molecules.
There are two types of MHC molecules, MHC class 1 and 2. All nucleated cells express MHC 1 molecules when infected (including T cells). Cytotoxic T (CD8) cells only recognise MHC 1 molecules. When cells are infected with viruses, they express MHC 1 molecules.This allows cytotoxic T cells to destroy infected cells.
MHC class 2 molecules are only expressed by professional antigen presenting cells (APCs) such as macrophages. APC capture extracellular antigens and present them via MHC 2 molecules. The presented antigen is recognised by Helper T (CD4) cells(cytotoxic t cells cannot recognise MHC 2 molecules because they don't have CD4 coreceptors).
Human immunodeficiency virus (HIV) can only enter cells with CD4 coreceptors, thus HIV can only infect helper T cells and not cytotoxic T cells. For this reason, it is often said that HIV patients have a decreased CD4+ count. As a result, the effects of helper T cells are significantly reduced.
On a side note, if the CD4 coreceptors are mutated, HIV may not be able to infect helper T cells. Such occurrences have been reported.
There are two types of MHC molecules:
All nucleated cells express MHC class 1 proteins. A sample of all proteins produced within the cell are 'sliced' into their constituents which are then presented by MHC-I receptors. This means that if a virus infects a cell, the foreign proteins the cell is induced to make are expressed on the cell membrane by MHC-I. This is important as antibodies can not cross cell membranes, therefore this is the method by which a cell signals that it is infected by a virus.
MHC II are the special molecules that are only expressed in antigen-presenting cells, which I think is what you are referring to. As neither CD4 or CD8 T-cells present antigens, they do not express MHC-II themselves. Instead they have receptors for MHCII, allowing them to interact with antigens presented by other immune cells.
If a virus infects T-cells, the viral proteins are sampled by the MHC-I pathway and presented via MHC-I on the surface of the infected T-Cell. The immune response then progresses as for any other virally infected cell, destruction by CD8 T-Killers that destroy the cell by non-phagocytic means.