Cross-Presentation – Antigen Processing and Presentation

00:01 So those are the two classically defined processes of antigen presentation.

00:08 The presentation of endogenous antigen to CD8+ T-cells using MHC Class I, and the presentation of exogenous antigen to CD4+ T-cells using MHC Class II.

00:25 But in fact, those pathways are not absolutely set, they’re kind of interconnected in you like.

00:30 An exogenous antigen can be presented using MHC Class I.

00:35 An endogenous antigen presented using MHC Class II, as we can see on this particular slide.

00:41 And this isn’t a rare event.

00:43 This is happening all the time because often you will want exogenous antigens to be presented to CD8+ T-cells and you’ll want endogenous antigens to be presented to CD4+ helper and regulatory T-cells.

00:58 So the endogenous and exogenous pathways are not completely separated from each other.

01:04 Endogenous antigens can enter the Class II pathway and be presented to CD4+ helper and regulatory T-cells.

01:12 And exogenous antigens can enter the Class I pathway and be presented to CD8+ cytotoxic T-cells.

01:21 Let’s just look at one example of cross presentation.

01:25 Here we have a virus infected cell.

01:29 And antigens are being processed and presented using MHC Class I on this virus infected cell.

01:38 However, you can have capture of these antigens, either infected cells or viral antigens that are picked up by the antigen presenting cells in the infected individual.

01:51 So for example a dendritic cell can pick up antigen, and these viral antigens can then enter the cytosol.

02:00 So they’ve been engulfed by endocytosis or phagocytosis, and normally they would be given to the MHC Class II pathway.

02:09 But now they’re being put into the MHC Class I pathway because the antigen escapes from the vesicles and enters the cytosol.

02:17 And then they can go through the normal Class I pathway using that immunoproteasome that we heard about.

02:24 And you get this cross presentation so that MHC Class I is presenting, using a dendritic cell, MHC Class I is presenting to virus specific CD8+ T-cells.

02:38 And you get co-stimulation and activation of these cytotoxic T-cells.