West Nile virus (WNV) is a human pathogen that is transmitted by mosquitoes, whi

Question

West Nile virus (WNV) is a human pathogen that is transmitted by mosquitoes, which inject the virus into the host while they are feeding. To study the immune response to WNV, mice can be infected by injecting 100 PFU of virus into the footpad. This generates a robust immune response 7 days postinfection in the popliteal lymph node (LN), located behind the knee, as indicated in Figure Q9.36A.

Popliteal LN cells are isolated from the mouse shown above at day 7 post-infection, and the T cells are stimulated in vitro with the WNV peptide P33 plus dendritic cells isolated from a noninfected mouse of the same strain. Five hours later, the CD8 T cells in the culture are analyzed for their ability to produce the cytokine IFN-?, and the numbers of IFN-?-producing CD8 T cells are quantified. As a control, T cells are also stimulated with an irrelevant non-viral peptide (ova) plus dendritic cells. The results are shown in Figure Q9.36B.

a) Why is the T cell response different between the two lymph node populations?

To identify the important antigen-presenting cell required to activate WNV-specific CD8 T cells in the popliteal LN, wild-type mice and Batf3-/- mice are each immunized with 100 PFU of WNV in the left footpad, as above. Previous studies have indicated that Batf3-/- mice lack one particular subset of conventional dendritic cells, known as CD8?+ dendritic cells (DC), but otherwise appear to have normal numbers and subsets of all other immune cell populations (e.g., T cells, B cells, macrophages, etc.). The results of this experiment are shown in Figure Q9.36C.

b) Name two possible functions of CD8?+ dendritic cells that could account for the results seen in the Batf3-/- mice immunized with WNV.

To distinguish between the possible functions of CD8?+ dendritic cells, another set of wild-type and Batf3-/- mice are immunized with 100 PFU of WNV in the left footpad. At day 7 post-infection, the left popliteal LN are isolated from the mice, and the CD4 T cells in these LN populations are tested for responses to a WNV peptide bound to MHC class II (peptide Y). As a negative control, an MHC class II-binding peptide from ova is used. These results are shown in Figure Q9.36D.

c) Do these results rule in/out either of your proposed functions of CD8?+ dendritic cells indicated in answer to part (b)? Why or why not?

Explanation / Answer

a.

When the antigen is injected, the route of dissemination of the antigen determines the response. In the given scenario, the injected antigen is taken up by the APC cells, or the antigen entered the nearby lymph nodes through the lymph. It is captured and processed by APC (dendritic cells most commonly), processed and presented to the lymphocytes in the regional lymph nodes. The antigen might not have disseminated much in the circulation, as a result of which the processing and presentation of antigen confined much to the near by lymph nodes.

In the lymph nodes, the T cells are activated when they are presented with processed antigen.

b.

The functions of CD8+ T lymphocytes are:

c.

The results confirm the function of CD8+ dendritic cells since the function of other DC subsets is normal.

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