Immunology of the bovine lung

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Coming next: How stress, handling and other factors influence the immune system

Bovine respiratory disease (BRD) is the most economically devastating disease to the cattle feeding industry. Preventing, managing and treating BRD starts with understanding the normal defenses of the bovine lung. 

Philip Griebel, DVM, PhD, Vaccine and Infectious Disease Organization, University of Saskatchewan, says the lung is the portal of entry for respiratory disease because most of the pathogens are airborne. “We have incredible systemic defenses, including neutrophils and other factors,” Griebel says. “The bacteria have to come in somewhere. For example, I think the Mycoplasma enters through the lung and then crosses over systemically.”

Initially, the mucociliary apparatus in the trachea is important for keeping dust and other pathogens from getting down into the lung, but once there, one of the major defense mechanisms of the lung is the alveolar macrophage. “On a cellular basis, the alveolar macrophage is the cell that seems to get hit by many viral infections because that cell is there to remove particulates if they manage to filter down that far into the lungs,” explains Chris Chase, DVM, PhD, South Dakota State University.

Some of the chemical factors, such as surfactants, and some of the naturally secreted antimicrobial peptides that are secreted by epithelial cells are also very important, adds D. Scott McVey, DVM, PhD, University of Nebraska Veterinary Diagnostic Center. “Anything else that comes into play is usually the result of the initiation of an inflammatory response. Deep in the lung, that’s the primary barrier against infection.”

Kip Lukasiewicz, DVM, Ainsworth Veterinary Clinic, Ainsworth, Neb., says the mucociliary apparatus prevents normal commensal bacteria from migrating down into the lungs. “But right now in the feedlot, we see a lot more infectious bovine rhinotracheitis (IBR) incidences where the little finger-like villi projections in the trachea are broken down,” he says. “When you compromise the normal defense mechanism of the trachea with a virus or dust, it allows bacteria to migrate down and overwhelm the lung’s defense and creates an inflammatory response.”

There is a very rapid recruitment of defenses during an insult, but the lung serves the physiological function of air exchange, and inflammation is counter-productive to normal lung function. “There are a number of chemical factors that rapidly inactivate agents like the host-defense peptides,” Griebel says. “Not only are they antimicrobials, but they’re also very anti-inflammatory. The lung is trying to maintain a homeostasis and avoid inflammation or the recruitment of these other defenses because that disrupts the function of the organ.”

Griebel adds that serum antibody is transported into the deep lung. “For a long time, we thought just the mucosal defenses like IgA were important. We now know that IgA may be important in the upper respiratory tract, but when we get into the lower respiratory tract, serum IgG antibody can be transported in. What’s going on systemically can also influence the local defenses as well.”

It’s believed there is an element of overload in many clinical infections. With bacterial pneumonia, there are a couple of things probably happening. “One is that there usually is an upper respiratory burst of bacteria that descends into the lung, carried on necrotic debris in mucus,” McVey explains. “Two, there is paralysis of the mucociliary apparatus. The bacteria can multiply very quickly, and there are only so many alveolar macrophages. The innate mechanisms can handle only so much, so they very quickly get overwhelmed.”

We have innate or non-specific immunity, and we have acquired or specific immunity, says Breck Hunsaker, DVM, PhD, Livestock Consulting Services and Horton Research Center, Wellington, Colo. “All of our focus, from a pharmaceutical therapeutic standpoint or even a biological preventive standpoint, is at the level of specific immunity or trying to fix what’s already been broken rather than maintaining and sustaining those innate physical non-specific barriers. By the time we intervene with therapeutics or focus on prevention through vaccination, we’re really focusing on that specific layer after the physical barriers  —  the non-specific innate immunity  —  have already been breached. Maybe we feel that’s where we can actually do something and make a difference. Stress factors are insults to that innate immune function, and we try to fix it with therapeutics or shore it up with vaccinations.”

Bacteria and viruses compromise defenses

Griebel has performed bacterial challenges following a primary IBR infection resulting in 50–80% mortality. “In contrast, if we take calves with healthy lungs and challenge them with massive doses of Mannheimia, then they completely clear the bacteria. These challenges are performed with a bacterial aerosol which goes into the lung but 24 hours later we cannot isolate bacteria from the lungs. The normal lung defenses are incredible and the alveolar macrophages have a remarkable clearance capacity, unless we compromise it.” That’s where viral infections become very important, he says, because respiratory viruses target epithelial cells, the mucosa, the first line of defense in the lung, and they start compromising that barrier function.

IBR begins in the upper respiratory tract, moves to the trachea and disrupts mucociliary clearance. Any virus that disrupts mucociliary clearance and the epithelial barrier then allows body fluids to start coming into the lung through leakage of serum and lyses of red blood cells. “Most bacteria require iron to grow and the leakage of serum proteins and red blood cell lysis provides an ideal culture media for bacteria in the lungs,” Griebel notes. “Then it doesn’t take a lot of bacteria to rapidly replicate.”

Under normal circumstances, without any inflammation, most of the bacteria are somewhat limited due to just the carbon sources that are available to them on the surfaces of the respiratory epithelial. But when you get the exudate of inflammation, McVey adds, there’s a lot of glucose, protein and a lot of lactate that the bacteria can use metabolically, providing substrate to expand bacterial numbers and biomass very quickly. “The inflammation component may be really important in the growth cycle for the bacteria. The inflammatory response recruits the neutrophils and I liken neutrophils to artillery. You may kill some of the enemy, but you’re going to tear up a lot of tissue in the process.”

Another big part of the inflammatory cascade is pH. “Mannheimia, Pasteurella and probably Histophilus are oxidative bacteria and their growth cycles depend on the presence of oxygen, and it’s somewhat pH-dependent,” McVey says. “They tend to expand very early in that infection and inflammation process, but as the tissue becomes anoxic and the pH drops, the conditions for bacterial growth are not quite as good. But the numbers have
already expanded to such an extent that it’s a done deal; we’ve created a biomass of bacteria and now the animal has to deal with it.”

Chase notes that in a study by Hurley, et al. (Journal of Animal Science) as acidosis or alkalosis is induced, the immune system goes south. “It works in a very narrow pH range,” he says. “When they’re loading up with grain, you have acidosis and that is going to contribute to the immune system not being very happy.”

The combination of disrupting clearance and setting up ideal conditions for bacterial replication is setting calves up for a fatal respiratory infection. “If we do a respiratory challenge with pure IBR virus, using a virulent clinical isolate, rarely do we see a calf die,” Griebel says. “In the feedlot, some people see fatal cases of respiratory disease which they think are purely IBR. But in our experimental models, virus alone rarely is fatal; it’s the secondary bacterial infection that’s the problem.”

Endotoxins

Mannheimia has a very potent leukotoxin, an RTX-type toxin that is rapidly toxic for ruminant leukocytes, which is important in its ability to resist clearance. “It can kill the alveolar macrophages and the neutrophils that come in,” McVey says. “It also amplifies the inflammation and the infectious, inflammatory response becomes a vicious cycle.”

Endotoxin (lipopolysaccharide) structures are very important, too. “It’s a double toxin whammy,” McVey describes. “The leukotoxin is a hard left jab and then the endotoxin is a right cross. Combined, you also start getting epithelial cell and endothelial cell damage in the blood vessels. There are a lot of somatic structures that are involved, so it’s a very complex pathogenesis.” It’s not as simple as having a handful of antibodies to neutralize one or two toxins. Often by the time it gets in the lung, the bacterial load is very high and it’s difficult for the immune system to clear it. “Efficacy is usually measured as a significant reduction in lung lesions, not an absence of lung lesions,” McVey says.

Currently, Pasteurella multocida has toxins not nearly as well characterized as Mannheimia. “The kinetics of the growth and the kinetics of recruiting the inflammatory response are probably different between all of those strains,” McVey adds. “Histophilus somnus is different and is hard on endothelial cells and you see hemotogenous spread of bacteria. They are biologically distinct.”

The primary viral respiratory infections can also change the response to endotoxin. “With our own work we observed that IBR infection causes the release of a lot of interferon gamma,” Griebel says. “This cytokine markedly increases the response to endotoxin. But if you look at respiratory syncytial virus, they’re finding it also increases the expression of receptors that recognize endotoxin.” These viral-induced changes in response to endotoxin may be an important part of the viral-bacterial synergy which results in fatal pneumonias.

“It’s not only the animal’s genetic capacity to respond to these bacterial components, but what’s going on in response to the viral infections which alters the animal’s capacity to respond to bacterial infections,” Griebel adds.

Griebel says there is a very tight time-dependent relationship between the viral infection and how it alters the animal’s response to bacteria, further complicated in the feedlot where coincidental or serial viral infections may occur. “We do very controlled experimental work by exposing animals to one virus at a time and then analyzing the interaction with one bacteria. I don’t know what might be happening with coincidental or serial viral infections  —  how this might be modulating the immune response to a bacterial infection. BVDV may be very suppressive but IBR is very immune stimulatory. I don’t know what the net effect of such a combined viral infection might be on the host’s response to bacterial infections.” Multi-sourcing cattle, commingling and bringing in multiple pathogens creates a very complex picture for the clinical veterinarian, Griebel adds. “It is very difficult to have a predictable outcome.” 


Role of inflammation in BRD

As veterinarians, when you open up an animal and look at the lungs you don’t know exactly where it is in the disease process. What’s going on in that animal’s immune system can also have a big impact on lung pathology. “What you’re looking at in the lung may not be just a result of the amount of bacteria, but also reflects how the immune system reacts to the infection in terms of the inflammation,” says Philip Griebel, DVM, PhD. Sometimes the amount of lung damage doesn’t appear to correlate with a fatal respiratory infection and there can be relatively little lung damage associated with an acute respiratory infection.

Kip Lukasiewicz, DVM, agrees and says, “It’s frustra-ting when you open up an animal and see 20% of the lung consolidated and compromised, but the rest of the lung is good, as opposed to the animal that has survived for a long duration with 90%+ damaged lung. We have to realize that inflammatory response often ends up killing the animal.”

Respiratory disease is a local event in the lung, but it can also involve a systemic response. “That’s why we observe fever and anorexia in these calves,” Griebel explains. “There may be a disconnect between the amount of lung damage and mortality when there is a severe systemic response such as endotoxemia or septicemia. It’s very important to look at this disease process from a whole-animal point of view.”

D. Scott McVey, DVM, PhD, adds that similarly in humans with Gram-negative pneumonia and septicemia, two of the things that predict outcome better than anything else are renal function and the ability to maintain normal glycemia. “A significant decrease in renal output is a product of shock and falling blood pressure,” he says. “The same is true with the ability to regulate blood sugar. In a lot of the bad cases, blood sugars go sky high and you have to treat them with insulin. When renal function begins to drop and blood sugar begins to go up, the survivability in those cases is very poor, even with extreme intervention in intensive care units. It gets to a point where systemically the effects of the inflammation are so great that, therapeutically, it may be beyond rescue.”

It appears the way we shut down inflammation is the key. “Often we use corticosteroids, which is a huge hammer as far as the immune system is concerned,” Griebel says. “It stops cell migration, the recruitment of cells to the lung that are going to help clear infection, and may also cause severe damage to the lung tissue.”

Corticosteroids may buy the animal some time, but what’s important in the feedlot is to follow those animals and monitor survivability in the long term. “I’ve heard feedlot veterinarians say that corticosteroids only delay mortality since these animals cannot clear the infections,” Griebel says. “If they’re dying within 24 or 48 hours of that infection because of systemic inflammation, you’ve lost them anyway. I think our current therapeutic tools aren’t sufficiently specific to shut down those aspects of the inflammatory response that are harmful to the host, without compromising immune defenses. In the end, the animal needs to clear the infection if it is to survive.”

Breck Hunsaker, DVM, PhD, agrees and says if there was a way to immunologically shift the emphasis in the lung from neutrophilic infiltration to more pulmonary alveolar macrophages and gamma-delta T-cells along mucosal surfaces that are antigen-specific, that would be beneficial. “There could be some opportunity with neutrophil degradation associated with consolidation and necrosis, as with some of the interferons and immunomodulators that we’ve played around with.”

The activation pathways are redundant, McVey says. If you knock one out, it may have an effect, but eventually the inflammatory and immune systems will catch up. “The idea of using corticosteroids with severe infection, septicemia and pneumonias is interesting. Publications on the human side have looked at that, and it’s a mixed bag. There are a small number of cases where it seems to really benefit the individual, and many cases where it does not, which indicates there might be a role for some kind of a compound like that if we knew specifically how to predict when to use it or what the mechanism was when it was working.”

Unfortunately, Griebel adds, we don’t have the diagnostic tools available in human medicine to help us understand if we are dealing with a systemic inflammatory response to septicemia during bovine respiratory disease.



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