1. IgA Background
IgA is secreted in mucosal tissue and is transported across mucosal epithelial barriers by the poly-Ig receptor
Poly-Ig receptor is cleaved; the part that stays bound to IgA is “secretory component”

2. Immune control of gut microbes
Hooper and MacPherson, Nature Rev Immunol. 10: 159, 2010

3. IgA in small intestines

4. IgA Background
Some IgA is T cell-dependent; some IgA is T cell- independent
Some IgA is produced in germinal center responses and exhibits somatic mutation
Deep sequencing of IgA reveals a diverse response that includes a few specificities produced abundantly

5. Deep sequencing of gut IgA+ plasma cells
Lindner et al. J Exp Med 209: , 2012
-Gut IgA plasma cells are highly polyclonal with a subset of highly expanded clones
-The frequency of somatic mutations goes up with age

6. IgA Background
Gut bacteria can be analyzed by flow cytometry and the bacteria that have IgA on them can be revealed by staining with a fluorescent anti-IgA: not all bacterial species are stained equivalently

7. Sorting IgA-coated bacteria
MICE
Palm et al. Cell 158: , 2014

8. IgA Question 1: What about gut bacteria induces an IgA response?
Formulate two hypotheses and
come up with one experimental approach to address one or both of these hypotheses

9. Question 1: What about gut bacteria induces an IgA response?
Approach 1: Controlled antigen delivery to the mucosal immune system
HA107 is an E coli auxotroph for m-diaminopimelic acid (DAP) and for D-alanine (required for peptidoglycan biosynthesis and not made by host)

10. Most IgA plasma cells in gut are dependent on gut bacteria
Hapfelmeier et al. Science 328: , 2010
HA107 gavaged 6 times over 14 days; IgA plasma cells (green) evaluated after 4 weeks total (blue: DAPI staining of nuclei)
(ASF: altered Schaedler flora, a mixture of 8 mouse gut bacteria that creates a relatively stable community)

11. Threshold for IgA response to E coli K-12
Hapfelmeier et al.
Science 328: , 2010

12. Question 1: What about gut bacteria induces an IgA response?
Hapfelmeier et al. Science 328: , 2010
Live bugs induce IgA >10X better than heat-killed bugs

13. Question 1: What about gut bacteria induces an IgA response?
Quantitative factors vs. Qualitative factors:
study a normal gut microbiota

14. Sorting IgA-coated bacteria
MICE
Palm et al. Cell 158: , 2014

15. Sorting IgA-coated bacteria
ICI: relative abundance IgA+/ relative abundance IgA-
Mice: 4 taxons enriched in IgA+ coated; 22 taxons enriched in IgA-
Palm et al. Cell 158: , 2014

16. Sorting IgA-coated bacteria: role of location in IgA induction
Bunker et al. Immunity 43: , 2015

17. Sorting IgA-coated bacteria from people with Inflammatory Bowel Disease
Palm et al. Cell 158: , 2014

18. Consortia of IgA+ vs. IgA- microbes
Palm et al. Cell 158: , 2014

19. Consortia of IgA+ vs. IgA- microbes: different degrees of IgA induction
Palm et al. Cell 158: , 2014

20. Consortia of IgA+ vs. IgA- microbes: invasion of the mucus layer by the IgA+ bugs
Palm et al. Cell 158: , 2014

21. Consortia of IgA+ vs. IgA- microbes: inflammatory action
Palm et al. Cell 158: , 2014

22. Microbiota: an important variable in animal research