Our overall goal is to understand class II MHC(MHC-II) antigen processing mechanisms and how they may be modulated under different circumstances. In general terms, these studies will address: 1. Poorly understood aspects of the major MHC-II mechanisms. 2. Additional recently discovered processing mechanisms. 3. The means by which these various mechanisms are regulated under different pathophysiologic conditions, e.g. infection. The studies are relevent to understanding host-pathogen interactions (e.g with Mycobacterium tuberculosis), as well as mechanisms of vaccine efficacy (e.g with regard to targeting immunogens to specific antigen-processing pathways for enhancement of protective responses and understanding the mechanistic basis of adjuvant activity, cf NIH PA-97-101. Aim 1. Characterization of distinct endocytic antigen processing mechanisms used for different epitopes and the relative roles of early endosomes and late endocytic compartments in these mechanisms. We will further explore mechanistic differences between late endocytic antigen processing mechanisms and an early endocytic processing pathway that was discovered in experiments funded by this grant during the previous funding period. Aim 2. To characterize the mechanisms of phagocytic antigen processing. We will apply novel technological approaches to isolating and analyzing phagosomes for antigen processing function. Aim 3. To determine the mechanisms by which Mycobacterium tuberculosis inhibits antigen processing. This may be of importance to understanding the maintainance of latent infection by this organism. Aim 4. To characterize the mechanisms whereby cholera toxin (CT) and E. coli heat labile enterotoxin (LT) affect antigen processing. These toxins produce both inhibiting and enhancing effects at different stages of antigen processing and presentation. These studies will shed light on both antigen processing mechanisms and the effects of these adjuvants, which may be used in future human vaccines.