The components of innate immunity can be thought of as being like players in a football team – all need to play together to achieve good results. This opening comment came from Professor Hans-Joachim Schuberth of Hanover University of Veterinary Medicine at a meeting hosted by Elanco Animal Health and attended by 150 veterinarians from Europe, Russia, Israel and the Middle East.
The Elanco Vital 90TM Days Conference took place in Barcelona in November with a particular emphasis on how veterinarians can work more effectively with dairy farmers to improve herd health and productivity. Innate immunity, which plays an important role in protection of the udder, is a key component in the mix of variables that veterinarians have to consider when looking at health initiatives in the dairy cow.
Acquired versus innate
A clear message to come out of Professor Schuberth’s presentation1 was that acquired immunity cannot be relied upon to provide a protective response against pathogen invasion in skin glands such as the udder. A mechanism for immunological memory is not known to exist in the udder and instead the organ has to rely on cellular and soluble defences of the innate immune system.
Cellular defences include epithelial cells, mast cells, macrophages and neutrophils; soluble defences include the cytokines, antibacterial peptides and chemokines. Most pathogens are eliminated very soon after contact and only a small number are ultimately dealt with by acquired, antibody-driven immunity in other organs.
Pathogens that persist beyond initial defensive responses are recognised by toll-like receptors (TLRs) on the epithelial cell. The most important response in innate immunity is release of antimicrobial peptides which can be thought of as the host’s natural antibiotic. Activation of TLRs triggers cells in the alveoli of the udder to switch from casein production to production of these antimicrobial peptides. If this switch does not occur quickly enough, host defences can be overcome.
Toll-like activation and tolerance
There are also toll-like receptors on cells of the innate immune system and there is an element of cross communication between these cells and the epithelial cells which aids in recruitment of soluble defences, such as the antimicrobials.
The toll like receptors can be up or down regulated according to other factors. Most specifically, pregnancy, an increase in plasma non-esterified fatty acids (NEFA) and negative energy balance, can all affect TLR activity, and therefore the magnitude of response.
In E. coli mastitis, endotoxins can play a role in the development of the condition. Endotoxin tolerance can however occur. In this instance, the first contact between an epithelial cell and the lipopolysaccharide toxin results in a transient, mild inflammatory response which then becomes silent. A second exposure resulting from pathogen produced endotoxin, where there has been previous priming, results in a reduced inflammatory response and tolerance.
In the experimental situation, priming each quarter of the udder with minute quantities of lipopolysaccharide toxin, followed by E. coli infusion, resulted in an absence of leukopenia, no or mild fever and reduced clinical scores. The bacterial load in the milk was markedly reduced compared to cows that had no pre-treatment with endotoxin. It is suggested that priming with endotoxin might enable animals to avoid dysregulated inflammation and allow the epithelial cells and innate immune defences, to remove the pathogen.
Neutrophils in action
In the ultimate phase of the innate immune reaction, neutrophil infiltration occurs. Neutrophils are released from bone marrow as short lived cells, surviving only a matter of hours but are ubiquitous throughout the body. The cells contain glycogen granules, used as fuel and lysozyme granules to kill bacteria. Being able to be fuelled by anaerobic glycolysis allows neutrophils to enter low oxygen areas of the body where there are dead and dying tissues.
The cells travel by rolling along blood vessel walls, loosely adhering to endothelial cells. Emerging neutrophils digest the adhesions between endothelial cells and squeeze through into infected tissue – a process known as diapedesis. Immigration of neutrophils is the hallmark of infection and while they destroy and phagocytose pathogens, they also generate toxic reactive oxygen species (ROS) that can destroy host cells too.
In E. coli mastitis, cows that respond with less severe signs, usually experience fast migration of neutrophils to the site of infection resulting in a more rapid reduction in bacteria. Those with severe signs show slower migration and diapedesis and an increase in bacterial numbers.
Around parturition, the function of neutrophils is compromised. Factors such as fat mobilization with increasing circulating NEFAs, low energy and glucose levels and increased cortisol, all reduce expression of the receptors and chemokines necessary for neutrophil adhesion, migration and phagocytosis. Both cytokine release and ROS formation is reduced.
Immune response and regulation
The mechanism by which the immune response is controlled is still considered complex. Lipoxins and resolvins are likely to play a key role in allowing macrophages to engulf apoptotic neutrophils. While macrophages control neutrophil lifespan, there are few macrophages in the alveolus of the teat, so other factors are likely to have an influence.
The role of lipoxins, resolvins and protectins is likely to be critical in determining the chronicity of infection. Lipoxins not only have a profound effect on neutrophils, being involved in chemotaxis, migration and adhesion but also release other cytokines, such as Interleukin 8 (IL8) from the epithelial cells. If the innate immune system is a football team, perhaps the neutrophil needs to be thought of as the manager, having a strong influence on which players are brought onto the pitch, the tactics used and the overall success of the team’s response.
In the context of The Vital 90TM Days conference, Professor Schuberth’s lecture emphasised the vulnerability of the dairy cow during the transition period. During this period there are significant risks from pathogens but great scope for the immune system to be compromised. Much of the work and focus of veterinarians has, in the past, centred round acquired immunity and vaccination but Professor Schuberth believes that we cannot expect good results with this approach when it comes to mastitis control because of the peculiar immune response in the mammary gland.
There is tremendous scope to do more to support the mechanisms of innate immunity around the transition period and to produce better outcomes for pathogen-driven diseases, especially for economically significant and difficult to treat conditions such as mastitis.
1.Schuberth HJ (2014) Innate immunity: introduction and the role and function of neutrophils, The Vital 90 Conference, Barcelona, Oral presentation.