COLLEGE STATION, TX – Humans have been raising cows for their meat, hides and milk for millennia. Now it appears that the cow immune system also has something to offer. A study of an extraordinary family of cow antibodies, led by researchers at The Scripps Research Institute (TSRI) and coauthored by three investigators from Texas A&M College of Veterinary Medicine & Biomedical Sciences (CVM), points to new ways to make human medicines.
The CVM’s faculty members’ expertise in immunology and infectious disease, as well as their easy access to a herd of cattle, made them a natural fit as collaborators.
“These antibodies’ structure and their mechanism for creating diversity haven’t been seen before in other animals’ antibodies,” said Vaughn V. Smider, assistant professor of Cell and Molecular Biology at TSRI and principal investigator for the study, which appears in the June 6, 2013 issue of the journal Cell.
Antibodies, large proteins in the immune system, resemble lobsters with a tail and two identical arms for grabbing specific targets, called “antigens,” often parts of pathogens like bacteria or viruses. At the end of each arm is a small set of protein loops called complementarity-determining regions (CDRs), which actually do the grabbing. By rearranging and mutating the genes that code for CDRs, an animal’s immune system can generate a vast and diverse population of antibodies-which, collectively, can bind to just about any foreign invader.
In humans and in many other mammals, most of an antibody’s specificity for a target is governed by the largest CDR region, CDR H3. Researchers have been finding hints that an unusually long version of this domain can sometimes be the key to a successful defense against a dangerous infection, such as HIV.
Waithaka Mwangi, Assistant Professor in the Texas A&M College of Veterinary Medicine and Biomedical Sciences (CVM) and an author on the Cell paper, suggests thinking of these long CDRs as a probe on a thin extended scaffold that can fit narrow crevices to reach and bind unique hidden pathogen determinants that ordinary antibodies cannot.
As Smider’s area of research includes finding new ways to generate therapeutic antibody proteins, reports of long CDR H3 use caught his interest. “We started thinking about how we could make these long CDR3s that are so rare in humans, and we knew from the literature that cows make even longer ones all the time,” he said.
Although the structure of the long CDR H3 protein in previous studies of the human anti-HIV antibody had seemed unusual, the corresponding structure in the cow antibodies turned out to be unique in the known world of animal antibodies: a long “stalk” element topped by an antigen-binding “knob.” Sequencing of the DNA that codes for the knob region revealed an unusual abundance of cysteine-a sulfur-containing amino acid that is apt to bond to a nearby cysteine on the same protein chain, thus forming a loop.
Analyses of these DNA sequences, some of which were conducted at Texas A&M, also indicated that, in the cow B-cells where these antibodies are made, the knob-coding gene segments are extraordinarily likely to develop point mutations that either add or subtract cysteines. The effect of these tiny mutations is to create or remove-often radically-antigen-grabbing loops on the structure.
In the cows, binding of these antibodies to viruses is almost entirely done by the knob on the long CDR H3, which shows that these antibodies do have an important function in the immune system. “For the very first time we have an ultra-long CDR3 antibody binding to an actual pathogen,” said Mwangi, an expert in immunology who completed the initial assays that determined the binding target for these antibodies.
One question that remains is why the cow immune system evolved to make such antibodies. Smider suspects that it has to do with cows’ unusual, four-chambered, grass-fermenting stomach, with its extensive collection of bacteria and other microorganisms. “If some of these escape from the stomach and get into the bloodstream or other tissues, there could be some pretty serious infections; so that’s our starting hypothesis for why cows have this unusual immune defense,” he said.
The stalk-and-knob structure of the CDR H3 loops on these antibodies, which resemble structures found in some insect poisons and other proteins, also suggest that they evolved to grab a particular type of target. “What comes to mind are ion channel or pore structures in the walls of cells,” Smider said. “In any case, we’re hoping to find out whether any of the structures targeted by these knobs exist on microorganisms that cause human disease.”
“Potentially, the outcome of this research is going to be huge,” Mwangi said, “not only for cattle but also for human health.”
Michael F. Criscitiello, Assistant Professor at the CVM and one of the study’s authors, said this was a wonderful chance to contribute to such a groundbreaking study, as researchers at the CVM had experience with-and access to-cows. The entire project was made possible through collaborations of various people and labs each contributing their expertise to add pieces to the puzzle.
“Such collaborations bring together specialists in diverse fields and certainly facilitate future research,” said Terje Raudsepp, Associate Professor at the CVM and another of the study’s authors. “This is expected to lead to new collaborative projects in the future.”
The study was supported by the American Cancer Society, National Institutes of Health, Skaggs Institute for Chemical Biology, Scripps Translational Science Institute, Texas A&M College of Veterinary Medicine & Biomedical Sciences, and United States Department of Agriculture.