Molecule of the Month: Secretory Antibodies

Not every antibody can fight every type of infection. When thinking of our immune system, we might think all battles are fought in our blood, but in fact, most of the antibodies we produce protect other parts of the body. These antibodies are known as secretory antibodies, and they are our first line of defense against mucosal pathogens. Two types, secretory immunoglobulins A (sIgA) and M (sIgM), play a major role in protecting the mucosal linings throughout the body. Amazingly, due to the large area of mucosal surfaces, we produce around 2-3 grams of these antibodies in our gut every day. These antibodies are essential to our immune response against mucosal pathogens like SARS-CoV-2 and intestinal bacteria. They also play a role in beneficial interactions with our commensal microbiome. Recently published research is showing that these friendly bacteria promote resistance to infection by competing with pathogenic bacteria for sites of colonization, producing inhibitory molecules, and preparing our immune defenses.

Given their unique job, secretory antibodies have specialized structures that allow them to withstand the harsh environment of our lungs, stomach, small and large intestines, biliary system, and genitourinary tract. Secretory antibodies build on the classic Y-shaped structure of a typical antibodies, but add a few unique modifications that assist in their function. To assist with recognition of pathogens, secretory antibodies often include many arms: sIgA (shown here from PDB entries 3chn and 6ue7) is composed of two (or more) Y-shaped subunits, and sIgM has five Y-shaped subunits. These many arms allow them to bind simultaneously to many sites on a pathogen, strengthening the interaction. A special J-chain holds them together and short extensions at the end of the chains associate into a beta-sheet arrangement similar to an amyloid fiber to further stabilize the entire assembly.

Secretory antibodies rely on a specialized protein to deliver them to the proper place. In the endoplasmic reticulum and Golgi, the polymeric immunoglobulin receptor binds to the antibody and assists with export out of the cell. Once outside, the receptor protein is cleaved, releasing the antibody. The antibody-binding portion of the receptor, called the “secretory component,” remains bound to the antibody, as seen in the structure shown here.

Bacteria fight back against these secreted antibodies. For example, Streptococcus pneumonia, a bacterial pathogen that causes pneumonia and meningitis, secretes a protease (IgA1P) that inactivates antibodies by breaking them into pieces. As seen in PDB entry 6xja, it consists of three parts: a domain that binds to the antigen-binding (Fab) portion of the antibody, a domain that binds to the constant (Fc) portion of the antibody, and a protein-cutting active site in between. Intriguingly, these bacteria then use the Fab fragments as a protective coat to hide from the host’s immune system.