Class II Antigen Processing
The recognition of MHC molecules loaded with pathogen-derived peptides by T cells initiates the immune response. My laboratory is interested in how human class II molecules acquire their antgenic peptide cargo. Specifically, we are interested in the role that 2 accessory molecules, HLA-DM and HLA-DO, play in the class II antigen-processing pathway. DM facilitates peptide loading of class II molecules, while DO specifically inhibits the the peptide loading ability of DM.
We believe that the interplay between these 2 molecules has a fundamental role in the modulation of MHC class II-mediated immune responses. The powerful and destructive nature of an immune response must be regulated in a way such that healthy cells are not destroyed during the process of fighting off disease. When the immune system is not correctly regulated, autoimmune disease can occur. The modulation of antigen presentation by DM and DO clearly present a method of controlling the immune response and preventing autoimmune disease.
The Class II Antigen-Processing Pathway
Over the past decade, the basic mechanisms involved in the generation of major histocompatibility complex (MHC) class II-peptide complexes have been worked out. The class II alpha and beta subunits assemble in the ER with another transmembrane glycoprotein called the invariant (I) chain. The fully assembled nonameric [(alpha-beta-I)3] complex exits the ER and moves through the Golgi stacks, where it is sorted by a signal in the cytoplasmic domain of the I chain into late endosomes and lysosomes. In these acidic compartments, the I chain is proteolytically cleaved from the class II molecules by resident proteases until only a small peptide fragment of the I chain (class II-associated I-chain peptides or CLIP) remains bound in the peptide-binding groove of the class II molecules.
Next, another class II-like molecule, HLA-DM (H2-M in mice), associates with the class II-CLIP complex, forcing CLIP from the binding groove of the class II molecules. DM remains bound to the "peptide-free" class II dimers, stabilizing them until appropriate antigenic peptides derived from engulfed extracellular antigens (such as bacteria) are generated and loaded onto the empty class II molecules. DM also "edits" the peptide repertoire, ensuring that only high-affinity peptides are presented on the cell surface.
The resulting class II-peptide complexes are released from DM and transported, by a poorly defined process, to the cell surface where they are presented to T cells. CD4 T cell recognition of such a complex results in an immune response to the engulfed antigen.
HLA-DO Inhibits DM-mediated Class II Peptide Loading
HLA-DO (DO) (H-2O in mice) is another class II-like molecule that is expressed in B cells and thymic epithelia but not in other APC (i.e., macrophages and dendritic cells). Association of the DO ab heterodimer with DM in the ER is required for DO transport to endosomal compartments, where DO/DM complexes accumulate. This suggests that DO plays a unique role in the class II processing pathway, specifically in B cells, either by regulating or altering DM function.
Our initial biochemical and cell transfection analyses clearly showed that DO inhibited DM-mediated peptide loading, resulting in a down modulation of the class II processing pathway. However, recent studies have suggested that the function of DO may be more multifaceted. Depending on both the experimental system and the Ag studied, it has been shown that DO can inhibit, promote, or have no effect on class II peptide loading.
Collectively, these studies suggest that DO modifies the loading of specific peptides on to class II molecules, which results in a change in the class II peptide repertoire. However, the biological relevance of a cell-specific modulator of DM function has proven to be enigmatic, and it remains to be determined if modulation of DM activity is the only or even the primary function of DO. Thus, the main focus of our lab is to determine the function DO/H2-O and to determine the functional consequence of DO/H2-O expression in vivo during the course of immune responses.