Quantitative Biology: The Next Generation: The alternate routes of allergic responses

As spring approaches for the northern hemisphere (or as a cat approaches from across the room), allergy sufferers might wonder about how their symptoms originate. In some ways, they're in luck. The molecules involved in allergic reactions have been studied for decades, and to some extent, we've developed a cohesive picture of how these molecules work together. However, the immune system is always full of surprises. Here are some newly-discovered and seemingly fundamental roles for proteins that we knew existed, but that rarely made appearances in review papers.

Background: The signals before the sneezes

In an article published in Science this February, Rivera and co-workers investigated how mast cells, which play a central role in allergic responses, can distinguish between different antigens. Antigens (also known as allergens) are molecules that bind to antibody-receptor complexes on the mast cell's surface. Antigen binding can initiate a process that leads to release of substances that induce inflammation and the symptoms of allergies. The two different antigens used in this study differ in their affinity, meaning how tightly they bind to receptors.

A typical view of signal initiation in mast cells via the high-affinity receptor for IgE, also known as FcεRI. The upper part of the image represents the space outside of the cell, and the bottom part represents the inside of the cell. Antibody-FcεRI (receptor) complexes are clustered by binding to an antigen. The kinase Lyn can then phosphorylate the receptor, meaning that phosphate groups are attached to multiple parts of the receptor. The phosphorylated receptor can bind another kinase, Syk, which goes on to phosphorylate multiple targets, including Lat.

Why does binding affinity matter? It has been proposed that antigens that bind more tightly, and stay in contact with receptors for a longer period of time, allow signaling to progress further and induce stronger cellular responses. One response that can be measured is overall receptor phosphorylation (see image), one of the earliest steps in signaling. The low-affinity antigen does indeed induce less receptor phosphorylation than an equal dose of the high-affinity antigen. However, if the amount of low-affinity antigen is 100x higher, total receptor phosphorylation is roughly equal. Which raises the question...

Are all responses affected in the same way?

The answer is no (which others have also found). One of the most important downstream players in this system is the adaptor protein Lat, which is phosphorylated to recruit an array of other signaling proteins. Lat undergoes less phosphorylation in response to the low-affinity antigen than the high-affinity one, even when receptor phosphorylation is equal. Surprisingly, the related but less well-studied protein Lat2 undergoes more phosphorylation in response to the low-affinity antigen. Lat2 phosphorylation depends, directly or indirectly, on a kinase called Fgr. Fgr's close relatives, Lyn and Fyn, are well-known for their roles in initiating mast cell signaling, but Fgr has largely gone under the radar.

A possible clue about the origins of these differences is that even when total receptor phosphorylation (the total phosphorylation of multiple sites) is equalized, the low-affinity antigen causes more phosphorylation of at least one specific receptor site. So although total phosphorylation is the same, the contributions of individual sites may be different.

Finally, the authors considered how the low- and high-affinity antigens influence the messages that the mast cell sends to the rest of the immune system. The two antigens caused mast cells to release different types of signaling molecules (chemokines vs. cytokines), which induced different types of immune cells to arrive at the site of inflammation. So it seems that the Fgr/Lat2 pathway elucidated in this paper enables responses to low-affinity antigens, but these responses are qualitatively different from those induced by high-affinity antigens.

What we can learn:

The idea of higher affinity -> more signaling -> stronger responses can explain some aspects of signaling, but is too simplistic to explain how specific responses are enhanced for low-affinity antigens.
Lat2 and Fgr may play important roles that are distinct from their more famous protein relatives, Lat and Lyn.
Several blanks are yet to be filled. Does Fgr act on Lat2 directly? How does the phosphorylation pattern of individual receptor sites differ with antigen affinity (although, that's likely to be experimentally challenging)? Although this system has been studied for a long time, there's evidently still a lot to learn about how quantitative differences between antigens lead to qualitatively different cellular behaviors.

References:

Suzuki, R., Leach, S., Liu, W., Ralston, E., Scheffel, J., Zhang, W., Lowell, C., & Rivera, J. (2014). Molecular Editing of Cellular Responses by the High-Affinity Receptor for IgE Science, 343 (6174), 1021-1025 DOI: 10.1126/science.1246976

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Liu ZJ, Haleem-Smith H, Chen H, Metzger H. Unexpected signals in a system subject to kinetic proofreading. Proc Natl Acad Sci USA 98:7289-94. (2001)