Viral antigen on an MHC Class II molecule?

[BlondeCookie]

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Immuno Q here.

I saw a diagram in First Aid that made little sense to me. A viral antigen can't be loaded onto an MHC Class II molecule of an APC, can it? I always thought ath viral or other intracellular antigens can only be loaded onto MHC Class I molecule, while bacterial or other extracellular antigens can only be loaded onto MHC Class II molecules. However, the diagram clearly shows a viral antigen loaded and presented by an MHC Class II molecule.

 

[TerpMD]

Are you talking about p194? I think that it is an APC (not an infected cell) picking up "extracellular virus" in that picture. Classically I always think of viral epitopes on MHCI also but I don't know if it is a hard and fast rule.

 

[opb]

The MHC I molecules are involved in the endogenous pathway of antigen presentation. So that's presenting anything that occurs normally within a cell (allowing the immune system to see those cells as "self"). Once a virus infects a cell, that virus becomes part of that cell. The virus expresses proteins and what not inside the host cell. These "endogenous" viral particles get packaged into MHC I molecules and presented. However, the viral particle in the MHC I is not a "self" antigen, and therefore a CD8 cell will recognize that cell as foreign and kill it.

The MHC II molecules are involved in the exogenous pathway of antigen presentation. They present extracellular antigens. MHC II molecules are on all APCs, so it's possible that an APC will recognize an extracellular viral particle, endocytose it, degrade it, package that onto an MHC II molecule which is then shuttled to the surface and thus presenting a viral particle in an MHC II molecule. These are recognized by CD4 T cells and involved in their activation.

Sorry if that was a bit overkill.

 

[ldhlsu1]

Dendritic Cell Macropinocytosis ?

 

[R_C_Hutchinson]

The MHC I molecules are involved in the endogenous pathway of antigen presentation. So that's presenting anything that occurs normally within a cell (allowing the immune system to see those cells as "self"). Once a virus infects a cell, that virus becomes part of that cell. The virus expresses proteins and what not inside the host cell. These "endogenous" viral particles get packaged into MHC I molecules and presented. However, the viral particle in the MHC I is not a "self" antigen, and therefore a CD8 cell will recognize that cell as foreign and kill it.

The MHC II molecules are involved in the exogenous pathway of antigen presentation. They present extracellular antigens. MHC II molecules are on all APCs, so it's possible that an APC will recognize an extracellular viral particle, endocytose it, degrade it, package that onto an MHC II molecule which is then shuttled to the surface and thus presenting a viral particle in an MHC II molecule. These are recognized by CD4 T cells and involved in their activation.

Sorry if that was a bit overkill.

Great explanation, thanks!

 

[osli]

Short version:

MHC I is going to display products made by the virus genome inside the host cell.

MHC II is going to display whatever it can grab as a virus drifts by, which for the most part means capsule or coat antigens.

 

[BlondeCookie]

The MHC II molecules are involved in the exogenous pathway of antigen presentation. They present extracellular antigens. MHC II molecules are on all APCs

Thanks for the help everyone. Just to clarify...

1. MHC I molecules are on ALL nucleated cells.
2. And is it also true that MHC II molecules are also on all cells that have MHC I?

 

[opb]

Thanks for the help everyone. Just to clarify...

1. MHC I molecules are on ALL nucleated cells.
2. And is it also true that MHC II molecules are also on all cells that have MHC I?

1. Yes
2. Somewhat. MHC II molecules are only present on antigen presenting cells (dendritic cells, macrophages, B cells, etc.). So ALL nucleated cells have MHC I. Only antigen presenting cells have both MHC I and MHC II.

 

[Loanna]

Just a little clarification ; MHC I are expressed on all cells EXCEPT RBC's.

 

[BlondeCookie]

1. Yes
2. Somewhat. MHC II molecules are only present on antigen presenting cells (dendritic cells, macrophages, B cells, etc.). So ALL nucleated cells have MHC I. Only antigen presenting cells have both MHC I and MHC II.

I understand what you are saying, but wouldn't it also be true that all nucleated cells have both MHC I and MHC II? Maybe my logic is screwed up here. Can someone help me out. This is my thinking...

If A = B = C, then A = C... so...

• MHC I is on all nucleated cells
• MHC II is on all APCs
• Therefore, if a cell that has an MHC cell is, by definition also an APC, then MHC II must also be on all nucleated cells. Correct?

If a nucleated cell has an MHC I molecule, it must also be an APC because it is presenting antigen to another cell. Am I wrong here?

 

[RxnMan]

The term "antigen presenting cell" refers to a cell that professionally chews up surrounding protiens and presents them to immune cells. This means things like macrophages, Langerhans cells, and so on.

Yes, a cell with a MHC I will be presenting antigens, but they will only be antigens from within it's own cytoplasm.

 

[MedLover25]

Just a little clarification ; MHC I are expressed on all cells EXCEPT RBC's.

it's not an exception. RBCs aren't nucleated (mature ones). So all nucleated cells have MHC I.

MHC II on APCs = DCs, MOs, B cells (and I believe platelets and one more cell type has them as well)

And the poster above is correct. APC is a term that refers to "Professional APCs"

 

[BlondeCookie]

it's not an exception. RBCs aren't nucleated (mature ones). So all nucleated cells have MHC I.

MHC II on APCs = DCs, MOs, B cells (and I believe platelets and one more cell type has them as well)

And the poster above is correct. APC is a term that refers to "Professional APCs"

OH! I get it now. Thanks. I didn't realize the term APCs only referred to professional APCs such as dendritic cells, macrophages, B cells, etc. In that case, not all MHC I cells are classified as an APC. Geez! Finally. I get it. I'm so dense sometimes. Thanks again everyone.

1. MHC I is on all nucleated cells 👍
2. MHC II is on all APCs 👍