MD, 1978, University of Rochester
Mechanisms to control display of foreign antigens to immune system
Our laboratory investigates the mechanisms which control the display of foreign antigens to the immune system and moregenerally the process of immune surveillance. In these processes the immune system uses MHC class I molecules to display on the cell surface oligopeptides derived from a cell's expressed genes. This allows cytotoxic T lymphocytes of the immune system to detect and eliminate cells expressing "foreign" sequences (e.g. from a viral infection or mutation). In many cases two distinct antigen presentation pathways are involved in the initial generation of the immune response and in the subsequent identification of the actual tumor or virally infected cell; these two pathways are termed cross presentation and direct presentation.
To initiate responses, antigens must be displayed on class I molecules of professional antigen presenting cells (e.g. dendritic cells). If these cells are not themselves making the antigen, they acquire them from dying cells and present them on class I molecules, through a mechanism called “cross presentation”. In this process tissue antigens are internalized into phagosome of the antigen presenting cells and then can follow two distinct pathways for presentation. We are studying how the antigen presenting cells and their cross presentation pathways work and might be exploited for immunotherapy.
Once cytotoxic T lymphocytes are stimulated they then seek out all cells that are synthesizing the "foreign antigen" and displaying its fragments on class I molecules. This display occurs through the “direct” or “classical” antigen presentation pathway. In this process, the majority of these MHC class I-presented peptides are generated by large proteolytic particles, proteasomes, which are present in the cytoplasm and nucleus of all Eukaryotic cells. These peptides must be of an exact size (8-10 residues) in order to bind to class I molecules. Where examined, the proteasome is responsible for making the proper C-terminal cleavage to produce antigenic peptides, however it often makes N-extended precursors. The precursor peptides can be trimmed to the mature epitope by aminopeptidases in the cytosol or endoplasmic reticulum or destroyed by other peptidases. After they are generated in the cytoplasm a fraction of the peptides are transported into the endoplasmic reticulum where ones of the correct size and sequence are bound by MHC class I molecules and transported to the cell surface for display. These processes determine whether antigens are recognized, the magnitude and specificity of the immune response and ultimately whether abnormal cells are eliminated. We are studying all of these processes in detail.
A related area of investigation concerns how the immune surveillance process is regulated. In order for productive immune responses to be generated, it is necessary for antigen presenting cells to acquire not only antigen but also to mature to an immunostimulatory state. The latter occurs when the antigen presenting cell senses that an antigen is dangerous, e.g. associated with microbial components. In addition, danger signals can be generated by injured and dying cells. We have found that when cancerous or infected cells die they not only release antigens but also endogenous adjuvants that markedly stimulate the generation of T responses. These adjuvants are ordinarily sequestered in the cytosol and are released when the plasma membrane loses integrity. One of these endogenous adjuvants has been identified as uric acid and data suggests that the active form of this molecule is monosodium urate crystals (MSU). There are also other endogenous adjuvants. We are actively studying these molecules, how they function and their role in immune surveillance.