NK cells and Cytotoxic T cell activation

[drmamaok]

I'm teaching a senior level immunology course and having to brush up on my Immunology. I have stumbled across a few questions that I've struggled to find answers to. If anyone knows and can connect me with some websites or publications regarding the following topics I'd appreciate it.

 

1) Why are NK cells classified as agranular leukocytes if they have cytoplasmic granules?

 

2) A text book I have says that a cytotoxic T cell can only be activated if it binds to Antigen on MHC from a professional APC (macrophage, dendritic cell or B cell). I was under the impression that any nucleated cell expressing MHC class I could present antigen to a Cytotoxic T cell to activate it. If not, why do nucleated cells express MHC Class I (besides self recognition for the Immune system)?

 

Thanks!

 

Also, if a CD 8 T cell can only recognize Antigen in MHC class I, then I assume antigen in the APC would be presented on MHC class I. If so, why does it need to be from a professional antigen presenting cell? Am I misinterpreting this text?

Sorry for the stream of thought. But I think I may have figured this out.

 

If a virus infects all cells, it will infect a professional APC and that APC will present Antigen on MHC Class I to a Cytotoxic T cell for activation.

 

The other nucleated cells infected will present Antigen on MHC Class I to the activated Cytotoxic T cell for the T cell to release granules and kill it.

 

If a virus does not infect all cells and does not infect an Antigen presenting cell, then the antigen must be phagocytosed from the mileu and it is presented on the APC by cross- presentation.

 

Correct??

 

I still would like to know about the NK cell as well!

[withoutid]

1) I am not so sure but activated CD8 T cells do have cytoplasmic granules and they are not classified as granulocytes.
2) A naive CD8 T cell must be activated by professional APC to undergo clonal expansion and gain effector state. Professional APC not only present Ag but also co-stimulatory molecules to the T cell (CD80/CD86-->CD28, OX40L-->OX40). In this licensing step, the T cells must see at least 400 Ag/MHC on a single professional APC (http://www.pnas.org/content/111/35/E3679.long). Once CD8 T cell are licensed by APC (mostly dendritic cells), they can recognized and kill virus infected cells with even 1 Ag on MHCI. Cells other than professional APC don't express co-stimulatory to license the naive CD8 T cells. They have MHC class I for one purpose: be killed by cytotoxic T cells once they get infected by virus or intracellular bacteria. Like you said, cross-presentation is a mechansim to effeciently present Ag by APC even when they are not infected by virus.