The MHCII Ag Processing and Presentation Pathway:
The recognition of MHCII molecules loaded with self and pathogen-derived peptides by CD4 T cells is an essential mechanism by which adaptive immune responses are initiated. Inappropriate recognition of MHCII-self peptide complexes by CD4 T cells can result in autoimmune disorders such as juvenile (Type 1) diabetes.
MHCII molecules acquire peptides from self and non-self proteins in endosomes of antigen presenting cells (APCs – dendritic cells, B cells and macrophages). The molecular pathways by which MHCII molecules acquire peptide cargo have been examined in detail (see Figure 1 for model of pathway). Briefly, newly synthesized MHCII ab heterodimers associate with the invariant chain (Ii) during their assembly in the ER. Ii occupies the peptide-binding groove of the MHCII preventing unfolded proteins from binding to MHCII molecules in the ER. Ii also targets MHCII-Ii complexes via the cell surface to late endosomal compartmentswhere Ii is degraded by resident proteases leaving only small fragments of Ii, class II-associated Ii peptides (CLIP) in the MHCII peptide groove. Exchange of CLIP for peptide derived from self-proteins and foreign antigens is directly catalyzed by the action of the MHCII-like molecule, HLA-DM (H2-M in mice). DM stabilizes MHCII, edits a subset of MHCII-bound peptides and is essential for MHCII Ag presentation.
HLA-DO/H2-O modulates peptide loading:
Peptide loading of MHCII molecules in the endosomes of APCs is modulated by another class II-like molecule, HLA-DO (H2-O in mice). DO associates with DM in the ER. DM is required for DO transport to endosomes where it accumulates as a stable complex with DM (Figure 2). We and others have shown that DO/H2-O modulates the peptide loading activity of DM/H2-M. It is now generally accepted that DO/H2-O alters the MHCII Ag presentation by reducing or modulating the complexity of the MHCII-bound self-peptide repertoire (Figure 3). DO/H2-O is down regulated upon APC activation, freeing DM/H2-M from DO/H2-O inhibition, presumably resulting in an optimally active MHCII peptide-loading pathway upon pathogen encounter in vivo. On the other hand, DO expression in non-activated APCs has been suggested to generate a broad, tolerogenic MHCII-bound peptide pool by dampening DM/H2-M activity. Thus, DO/H2-O expression potentially has an important role in autoimmune diseases such as Juvenile Diabetes by promoting central and/or peripheral tolerance. Until recently data supporting this idea was lacking.
Our studies showed expression of DO is tightly regulated during the germinal center response and that, counter to previous reports, DO was expressed in dendritic cells in addition to B cells. Over-expression of DO showed that it altered antigen presentation, but, frustratingly, presentation of only a subset of antigens was impacted. Even when presentation was altered, the change tended to be subtle. Recent data from our lab, however, now shows that modulation of the MHCII antigen-processing pathway by DO can have profound immunological effects. In particular, we showed that DO expression shapes the overall MHCII-self-peptide repertoire and, in doing so, promotes T cell tolerance. Indeed, our studies have shown that in NOD mice, a mouse model for Juvenile Diabetes, autoimmunity can be directly controlled by the activity of DO on antigen presentation (Figure 4). We believe that this represents a conceptual shift in our understanding of immunological tolerance.
Our studies have also shown that H2-O expression influences the ability of antigen specific B cells to compete for T cell help. Using a competitive adoptive transfer system we showed that antigen specific H2-O-/- B cells preferentially populate the germinal center in comparison to wild type B cells when challenged with antigen. Collectively, our studies support that H2-O expression in B cells dampens presentation of antigen-derived peptides, making it harder for the B cells to receive sufficient T cell help that is necessary for a B cell to enter the germinal center reaction. Therefore, H2-O directly controls the quality of the B cell response.
Overall, our studies have revealed that the processing and presentation of antigens is, paradoxically, fundamental for both effective immune responses and for tolerance to self. Furthermore, our studies have highlighted how subtle differences in the level of MHCII presentation can have profound biological consequences.
Research Goals: The NOD mouse model we have developed provides us with a unique opportunity to determine the mechanism by which DO modulates MHCII peptide presentation. Furthermore, this system has given us the opportunity to discover the importance of various islet autoantigens in initiating diabetes development. In addition we will further probe the impact of H2-O on MHCII peptide presentation by exploiting the antigen specific B cell transfer system we have developed. Finally, our ultimate goal is to determine the molecular mechanisms by which DO functions in modulating DM function in vivo.
Specific recognition of MHC-peptide complexes results in extraordinary T cell proliferation, with doubling occurring approximately every 2–4 hours. Data from my lab suggested that a poorly characterized PCNA interacting protein, p15PAF, might play role a role in this rapid expansion of antigen specific T cells. Intriguingly, p15Paf, has also been shown to be substantially up-regulated in virtually every type of cancer and studies support that p15Paf is itself an oncogene.
To determine the in vivo function of p15Paf in the immune system, my lab generated p15Paf deficient mice. Genetic disruption of p15Paf by homologous recombination resulted in altered hematopoietic stem cell (HSC) function and subsequent progenitor development, directly impacting the peripheral T and B cell compartments. Moreover, p15Paf controls the regulated proliferation of HSCs, keeping long-term-HSCs quiescent and protecting them from premature exhaustion. Our results also show that p15Paf is necessary for the survival of Lymphoid primed multipotent progenitor and common lymphoid progenitor populations (Figure 5).
p15Paf is, therefore, a key regulator of HSC quiescence and multipotent progenitor differentiation and development. Unlike many factors that have been shown to control HSC development, p15Paf does not appear to be a transcription factor, suggesting it works via a novel mechanism. Indeed, recent microarray analyses of p15Paf deficient and wild type long-term HSCs suggest that p15Paf controls hematopoiesis via novel mechanisms. Identification of how p15Paf functions to maintain effective hematopoiesis will likely yield information relevant for the treatment of cancers such as childhood leukemia and lymphoma.
Research Goals: Collectively, our studies have established an important role for p15Paf in HSC development and function. Our long-term goals are to define the molecular mechanisms by which p15Paf functions in regulating hematopoiesis, immune cell development and function and tumorgenesis.