Joan was miserable. She had had such a great weekend with her friends on the beach at Rocky Point, and now she could hardly leave the bathroom she was so sick - vomiting and diarrhea and abdominal cramps. Thank goodness her roommate was fine - if Joan didn't get better soon, she was going to need a ride to the ER! She thought she had been careful about what she ate and drank, but something had definitely disagreed with her.
Joan had ingested E. coli O157H7, a strain of E. coli that produces an exotoxin that affects intestinal cells. (http://www.cdc.gov/ncidod/dbmd/diseaseinfo/escherichiacoli_t.htm). E. coli is a Gram negative motile rod that thrives in the intestine. Everyone has E. coli as normal flora, a strain that does not produce toxin but does make colicins, secreted proteins that inhibit the growth of other bacteria. E. coli is covered in short fimbriae (see electron micrograph above) that have strain-specific attachment proteins called adhesins. Other surface antigens are the O lipopolysaccharide portion of LPS (O157 means it was the 157th version of this antigen discovered) and the flagellar protein H. O157H7 is the serotype of this E. coli, meaning that it will induce antibodies to the O157 antigen and H7 antigen as well as other antigens. E. coli O157H7 is usually acquired from eating hamburger that has not been completely cooked, but it can also be ingested with uncooked vegetables like sprouts.
E. coli colonizes the surface of the large intestine and produces its exotoxin protein (Shiga toxin). This location is "outside" the body where there are no leukocytes, but specialized cells called M cells take up some of the E. coli and pass it through their cytoplasm into the deeper layers of lymphoid tissue in the gut (see Parham Figure 1.12 page 15). Damage to the intestinal mucous membrane by the toxin also allows E. coli and toxin molecules to come in contact with lymphoid tissues.
1. List from the information above the antigens on E. coli and what kind of molecules they are (chemically). What other molecules on or in E. coli are probably also antigenic? Explain the relationship between the O157 epitope and the LPS molecule. For diagrams of LPS, see below. The diagrams are in the same orientation, with the membrane attachment end Lipid A towards the right. The yellow boxes are sugars.
2. Describe one antibody molecule that could be produced to bind to the O157 antigen. Include in your description the names of the heavy and light chains, the definition of Fc and Fab regions, the isotype of the Ig molecule, and a description of the antigen-binding regions. What would be the difference in the antibody molecule specific for (capable of binding) O157 and another antibody specific for adhesin?
3. A typical antibody response to E. coli would be polyclonal. Explain what that means in terms of the antibodies that you would find in the circulation and in terms of the B cells that would respond to E. coli.
4. Describe the genes for heavy and light chains in the mature naïve B cell which is waiting in the mucosal lymphoid tissue (MALT) to recognize and respond to E. coli O157 epitope. Explain which gene segments encode the variable and constant parts of the H and L chains.
5. Explain the process of somatic recombination for H chain that resulted in the mature naïve B cell above. Include the names of the enzymes required for this process and explain how this process generates B cell antigen-recognition specificity (each B cell recognizes one antigen epitope) and diversity (the B cells in the MALT have thousands of different antigen specificities).
6. List the steps in B cell development. Include the Ig proteins produced at each stage and explain how only non self-specific B cells with functional membrane Ig emerge from the bone marrow.
7. Describe how a single B cell can have both IgD and IgM receptors with the same antigen-combining specificity but different Fc regions.
8. Discuss how isotype switching by a B cell following antigen stimulation is similar to and different from somatic recombination and alternative mRNA splicing. Given that E. coli is infecting the intestinal mucosa, what isotype of antibody would be most useful? What could those antibodies do to fight Joan's infection?
Not to scare you, but this case and the next deal with molecular information that students find most challenging in the course. Feel free to contact me or come see me if you need help.
Answer the questions with as much detail as possible and save as a Word document entitled YourNameCase3.doc. Send to jdecker@u.arizona.edu as an attachment by 9 AM Tuesday June 15. Make sure your name is in the document as well as in the title.
Supplementary Materials: Antigen, Antibody, Antibody Genes, B Cell Development,