Microbiology 204

Discussion Session #4Oct 31st, 2016

1-2:30PM Room: HSW1546

Faculty leader: Julie Zikherman

Ubelhart R, Hug E, Bach MP, Wossning T, Minden MD, Horn AHC, Tsiantoulas D, Kometani K, Kurosaki T, Binder CJ, Sticht H, Nitschke L, Reth, M, Jumaa H. Responsiveness of B cells is regulated by the hinge region of IgD. Nature Immunol. 16: 534-543, 2015

Everyone should read this paper before coming to class. You do not need to look at the Supplemental Material file.

LEARNING OBJECTIVES:

To understand mechanisms involved in B-cell receptor signaling in IgM+ versus IgD+ cells.

STUDENT ASSIGNMENTS

1. Discussion leader (see instruction page for advice on leading the discussion). Please note that this paper has a TON OF DATA!! You do not need to go over every panel in detail. It is fine to pick out a few figures and just make sure the general point is conveyed to the class.

2. Compare the structure of the IgM versus IgD BCRs and some older theories on why B-cells have both of these Ig on their surface. See Geisberger et al Immunology 118:429, 2006. One reason for the different signaling properties may be explained in a recent article from the same general group – see Maity et al Sci Signaling 8:ra93, 2015. Compare contrast B1 versus B2 B-cells. This review may help: Garraud et al BMC Immunol 13:63, 2012.

3. In Fig 1, the authors use triple deficient B-cells to reconstitute IgM and IgD BCRs. Why use these cells? Why do they have to be triple KO for all these molecules to serve as a “blank” cell for transfecting in IgM or IgD?

4. The authors say very little about the light chains (LCs) used in this study. What are they? Which LCs are most commonly used in mouse B-cells and how does this differ from human B-cells?

5. Figure 2 demonstrates that the longer the hinge linker region between the antigen binding Vh-Ch1 domains and the C-terminal Ch2 constant domains, the more capable the BCR (either IgM or IgD) is of signaling. Linkers of intermediate length, like the SG5 linker, have intermediate signaling strength to low valency ligands. Yet everything works with multivalent ligands. Can you speculate why this would be? It must be how conformational changes induced by binding the ligand are transmitted to the cytoplasmic domain of the BCR. How could this affect signaling?

6. Technical question: What is the difference between Fig. 3c versus Fig. 3d? Why show both?

7. Fig. 5f is a very simple experiment that shows the relative importance of IgM versus IgD in B1 cell development. Review this simple experiment and explain why it works.

8. Fig. 7 shows that IgM deficient mice produce fewer IgG natural antibodies against oxidized lipids antigens. Is this because the IgM deficient mice just have fewer B cells in general or is there something more specific about the effect of IgM deficiency on B1 versus B2 cells. How does Fig. 7b help answer this?

9. Review the hypothesis presented by the authors in the Discussion, for why there is a selective advantage for B-cells to express both IgM and IgD BCRs. How might downregulation of IgD help with B-cell selection in the germinal center?

Student assignment #s

1.Joe Hiatt10.Camille Simoneau

2.Theo Ross11.Patrick Schupp

3.Tina Solvik12.Elise Wollf

4.Ariane Panzer13.Cindy Tian

5.Germain Yong

6.Didi Zhu

7.Jeanmarie Gonzalez

8.ParinazFozouni

9.PriyaMohindra

(NOTE : if you will miss a discussion session, inform Dr. Lowell in advance and you won’t be given an assignment that week; if assignments have already been made, you should make a trade with one of your classmates who does not have an assignment that week so that your assignment is covered).