Dr. James Evermann
Lecture 9: Basis for Immune Resistance in Livestock (continued)
Bifurcation of the Immune Response
Cellular immunity (T cell mediated) is primarily directed toward endogenous or cell-associated antigen (viruses) and bacteria that are sequestered within cells of the body. CMI is directed to target cells in the body which have altered cell membrane antigens i.e. a virus infected cell. The virus growing inside the cell causes the expression of antigens on the cell's surface. These viral antigens are recognized by the T-cells as "non-self" and the T-cells are stimulated into action. Consequently, this is a receptor mediated function. If this recognition of "self" versus "non-self" fails it results in autoimmunity. Autoimmunity is where the T-cells start killing normal cells. Systemic lupus is an example of an immune mediated disease.
As inactivated vaccines do not stimulate T cells, sometimes the vaccines don’t work as they are not stimulating both aspects of the immune response.
In addition to the B cells and T cells the body has also developed an immune response that protects the mucosal surfaces of the respiratory tract and the gastrointestinal tract.
Border disease infection in utero causes Hairy Shaker syndrome resulting in lambs with hair like fleece and without a thymus. Without a thymus the lamb does not have cell-mediated immunity to disease, and will die within two weeks. Additionally, these lambs are usually born without a brain.
All the captive Cheetahs in the United States can be traced back to a defined parentage. Due inbreeding in this captive population, they lack hybrid vigor. They don’t have proper T-cell function or B-cell function, and are highly susceptible to infectious diseases. Infections that the normal cat would overcome, such as feline herpes, causes serious disease. These Cheetah cubs may die from infections because they have an impaired immune response. This is another example genetic impact upon the immune response. This is a combined immune deficiency, like occurs in foals. These animals appear normal at birth, but within about two months they develop a secondary bacterial or fungal infection. Infections that an immunocompetant foal would be able to resist, CID foals cannot fight off. They usually die of pneumonia within about 2 to 3 months. This is a genetically acquired deficiency which causes them to be more susceptible to infectious diseases.
Humoral immunity - functional aspects
There is a susceptible period in neonatal and pregnant animals during which the immune response is compromised. These are the two most susceptible periods in life for acquiring infection.
Neonates have not developed their immune system adequately so they rely upon maternal antibodies for protection. Neonates require protection anywhere from 4 weeks up to 6 months. There are examples of artificial animal models created in a laboratory in which animals are breed that don’t have any immune response. An example of such a genetic mutant is the nude mouse. These mice don’t have any hair and have no immune response; no T-cells, no B-cells. They have to be kept in a notobiotic (germ free) environment.
Lymphoid organs are distributed throughout the animal’s body. The main lymphoid organ are salivary glands, thymus, spleen, and bone marrow. Virtually every body system has lymph nodes in it. There are lymph nodes in the genitourinary system, the intestines, mammary glands, and the respiratory tract. Lymph nodes contain both B-cells and T-cells.
Lymph nodes regulate the production and differentiation of lymphocytes into B-cells or T-cells. Lymph nodes give rise to lymph which is a fluid containing lymphocytes that circulates throughout the body in lymphatic vessels. There are B-cells and T-cells resident in lymph nodes, and there are circulating B and T cells. So you can see that the immune response covers just about anything the body comes in contact with. The reaction time of the body differs if it is seeing an antigen for the first time or if it has seen it before. The time course differs following the first and second dose of antigen. Virgin B cells produce primed B cells and eventually memory B cells on subsequent exposure to antigen. This is called an anamnestic response.
Phagocytosis is the process of capturing and destroying foreign material (antigen). Neutrophils and macrophages are the phagocytic cells. Phagocytes are scattered along the respiratory tract and the digestive tract because those are the areas where there is the greatest exposure to microbes. Both of these tracts are exposed to the external environment, either through breathing or ingestion. Phagocytes play a valuable role in immune defense by either directly attacking and destroying antigens, such as bacteria, in tissues and in blood or by capturing antigens and processing them. Bacteria in the blood is referred to as bacteremia or septicemia. These two terms are interchangeable. Antigen is any foreign material that is recognized by the immune response following antigen processing by the macrophages. Processing means that the whole antigen is broken down into smaller antigen fragments within the macrophage. These fragments are then delivered to either the B-cell population or the T-cell population. Either antibodies or killer T cells are formed.
Macrophages are an important link between natural immune defense and acquired immune defenses. When a macrophage encounters an antigen it can either kill it, or can process it and get it ready for the acquired immune response. With the natural immune defense the antigen enters the body and the macrophage destroys it. The major difference is that there is no memory generated with a natural immune defense. The macrophage does not process antigen and present it to the B or T cells, it simply destroys the antigen so the B and T cells never even see it. Contrast this to the acquired immune defense where the macrophage encounters the antigen, processes it, and presents a fragment of it to the B-cells and/or the T-cells. The B and T cells react to the antigen and consequently generate memory. What is meant by memory is that when that B-cell or T-cell sees the same antigen again, it kicks into action immediately, within hours. If a B-cell or T-cell becomes alerted, they are referred to as being sensitized. This is an important concept to recognize. The macrophage serves as both natural immune defense as well as acquired immune defense. The acquired immune defense also has memory associated with it.
Antibodies are also called immunoglobulins, abbreviated Ig. There are four major classes of immunoglobulins: IgG, IgE, IgA, and IgM. IgG is the most abundant immunoglobulin in serum and colostrum, and is long-lived, greater than 6 months. Following antigenic stimulation, the first immunoglobulin produced is IgM. IgM lasts for about 10 days. So IgM is the early antibody and it is short-lived. IgG production occurs following IgM production. IgG is long lived and is produced in high levels in the plasma.
All mucosal surface; eyes, nasal cavity, oral cavity, alimentary tract, gastrointestinal tract, urogenital tract; are protected by the secretory antibody, IgA. Which immunoglobulin is the first line of defense against rotavirus and corona virus in the gut of a calf? This is a mucosal surface so the best protection is provided by IgA. To prevent these 2 diseases (rotavirus and coronavirus) you vaccinate orally. Oral vaccinations are sometimes better than parenteral inoculations, (those given in the muscle, or subcutaneously). Parenteral innoculations stimulate high levels of IgG while oral or nasal innoculations stimulate high levels of IgA, secretory antibody. Different types of vaccinations stimulate different classes of antibody.
IgE is the immunoglobulin that responds to antigens called allergens. IgE is responsible for the typical allergy reactions such as hives, asthma, or hay fever. In some cases, individuals that are predisposed to allergic responses can be sensitized by low doses of allergens.
Complement, abbreviated C', is a protein complex consisting of enzymes. Complement helps antibody react with antigen to destroy cells or bacteria. Complement is produced early in the immune response, during the production of IgM. So early in the course of an infection both IgM and complement defend against bacteria.
Cytokines, also referred to as interleukins,. are regulatory proteins that are secreted primarily by lymphocytes to help the interaction between lymphocytes and macrophages. The macrophage/lymphocyte interaction is very important for acquired immunity. There are a couple of interleukins that are especially important; IL-2 and IL-12. IL-2 and IL-12 stimulate key cytotoxic cells. Recall that inactivated vaccines only stimulate half the immune response; the B-cell response. If a killed vaccine could be manufactured that includes IL-2 and IL-12, then we would have a killed product that stimulates both B-cells and T-cells and diminish vaccine failures. That is the current direction that vaccine production is going. Unfortunately, the half-life of interleukins in the body is only about 5 minutes and they are very expensive.
Interferon is important during the first 24 hours of an infection.
Failure of passive of transfer occurs when neonatal animals fail to absorb adequate levels of antibody from colostrum or they ingest poor quality colostrum. If a young animal dies of septicemia or viremia, the first thing you should look for is not to find out what the virus or bacteria was, but if the animal had a failure of passive of transfer. If you have a calf that dies in a calf hutch and in an attempt to find out why it died, you open the gut and find E. coli. You open the respiratory tract and find Pasteurella . Why did that animal succumb to the E. coli and the Pasteurella infections? What allowed those infections to become virulent and cause disease and death? The underlying cause could be a failure of passive transfer. That is why it is important to check for a failure passive transfer. Knowledge of why passive transfer failed is a management tool because it will tell you if you have a dam producing poor colostrum or if this calf was not monitored closely enough to ensure it ingested sufficient quantities of colostrum. Just knowing what the virus or bacteria was that killed the animal isn’t going to give you the answer to your question. You want to know why this animal was sick or why it died and if failure of passive of transfer was the reason. There might be 20 or more other causes to consider. Failure of passive transfer is a reasonable initial rule out.
Make sure that you understand local immunity versus systemic immunity. Systemic immunity is in the blood. Mucosal immunity is local immunity that protects surfaces.
Primary and secondary antibody response.
Polyclonal antibody is a term used to describe a mixture of immunoglobulins to a wide array of antigens. Multiple B cells may produce multiple forms of immunoglobulin to antigens with minor differences in the surface protein structure, the epitope. The epitope is the specific binding region on the antigen where the antibody attaches. Each antigen has a specific epitope; that is how the antibody recognizes it.
Neutralizing antibody is the primary antibody that takes care of viruses. The virus is not destroyed. It is still intact under a coat of neutralizing antibody but it is not considered a threat. To protect against viruses, antibodies must be produced that attach to protein structures on the surface of the virus. The antibodies bind to the epitopes.
Monoclonal antibody - term used to describe a homologous Ig produced by a clone of B cells. This occurs naturally in neoplasia of B cells resulting in myeloma.
Hybridomas - Myeloma cells cultured in the lab survive indefinitely. Myeloma cells can be fused with a normal B cell. The resulting hybridoma produces large quantities of monoclonal antibody (M-Ab) which have tremendous value in the prevention of certain diseases such as GenecolŪ for E. coli infection in calves. M-Ab can be used in diagnostic assays due to their high specificity for particular antigens.
T cell functions have been more difficult to measure due to the requirement for living lymphocytes in bioassays. T cells function as either T helper (TH) cells or T cytotoxic (TC) cells. Our knowledge regarding T cell function has increased over the past 5 years. TH and TC are intimately involved with the major histocompatibility complex (MHC). TC cells destroy target cells that are carrying foreign proteins on their surface in a single "lethal hit." TC cells contain perforins and cytotoxins, which serve to destroy target cells. TC cells survive the target cell destruction thus can kill multiple target cells. Recall this is also the leading cell against tumors – the T-cytotoxic cells.
Immunosuppression - any failure of the immune response (IR) usually becomes apparent through increased susceptibility to infection and disease. Deficiencies can be genetically acquired or the result of infections that subsequently result in immunosuppression. As mentioned earlier, age has a profound effect on the functioning IR. Immunosuppression is any barrier of immune response that enables animals to become more susceptible to infection and disease. Recall the deficiencies can be acquired genetically or as the result of an infection.
You can begin to see where these deficiencies would have a profound influence on the survivability of the animal because the macrophages are very important in both the acquired immune response and also the innate immune response.
These are young animals and young children that have no B-cells or T-cells. Just for your own information, there is selective immune deficiency syndrome in which you can have either B-cell or T cells.
Secondary immune deficiencies. They primarily act upon T-cells.
Question: When you talk about the most susceptible time for the immune response and
immunity is during pregnancy, does that pertain to the fetus or for the dam?
Answer: For the dam. During pregnancy the animal is susceptible to infection because in
order to allow the fetus to develop, her immune response is compromised. Primarily the top
half of that chart that I showed you of the T-cells. This is the same in all species.
That’s why humans are susceptible to things like toxoplasmosis during pregnancy
because normally in a non-pregnant woman, they are able to overcome those infections. But
in pregnancy, their T-cell response is immunocompromised to allow the fetus to develop and
that’s what makes them more susceptible to infection and disease.
The neonate is just as susceptible to infection and disease because their immune system
has not been generated yet. It is still in the process of developing.
Question: Do older animals have higher levels of IgG? What about the levels of IgA?
Answer: The older the animal becomes, the more its immune response is active and the
higher the levels of IgG.
That is not necessarily true of IgA. IgA requires continual stimulation because it is a
mucosal antibody. It only responds half the time to the antigen within its area. So it is
not continuous. That is why animals become re-infected with the GI tract or the respiratory
tract, because it is a short-lived antibody. It does not matter how old they are. They
could be 4 weeks old or they could be 4 years old, it is still a short-lived antibody.
Question: Can you develop colostrum with IgA?
Answer: Yes, it is still short lived. That is the problem with trying to immunize pregnant
animals who develop high levels of IgA, it doesn’t get any worse, but it is still
short-lived.
Question: Does failure of passive transfer come from the mother producing poor quality
colostrum or does it come from failure to suckle?
Answer: It can be either the failure to ingest or poor quality colostrum. A first half
heifer or a mare foaling for the first time is going to have low levels of IgG in her milk
because she probably has not seen as many antigens as a 4 or 5 year old. So the older the
animal is, the more antibodies she has, and the better quality of their colostrum. There
is also an indication that some breeds of dairy cattle have poor levels of colostrum. I
have heard that Holsteins, for instance, might have a poorer level of colostrum than say a
Jersey. And so therefore, with Holstein calves, it doesn’t hurt for you to augment
their colostrum with another source of colostrum. That is strictly genetic on the part of
the dam, the calf can suckle all it wants and it still will have insufficient passive
transfer. So, poor quality colostrum can be lack of exposure of that animal, or it can be
genetic, or the calf does not ingest enough. So there are multiple reasons for failure of
passive transfer.
Questions: So if a Heifer calves about the same time as a 5-year-old or a 3-year-old
calves, would you give all the calves the 5-year-old's milk and disregard the heifer's
milk or are there other options to just wasting the heifer's milk? Does it even
matter what kind of colostrum the calf gets as long as it gets in with in 24 hours?
Answer: First of all, it does matter what kind of colostrum a calf receives because you
want to make sure it’s immune colostrum. Colostrum coming from an older animal could
enhance the quality of that colostrum. I don’t know if the nutrients change that much
between age, but certainly the amount of antibodies within the colostrum change. Instead
of throwing out the heifers colostrum you could use it for pooling. You
basically mix it with higher quality colostrum so it is not wasted but the pooled
colostrum is of lower quality than the unpooled 5 year old cows colostrum. Overall,
if you can, give the highest quality colostrum available so if you have to choose between a
5 year olds and a heifer's, go with the five- year-old's colostrum.
Question: How effective are bottles of IgG versus finding colostrum.
Answer: It is effective. It is very costly. The choice to use bottled IgG versus colostrum
should partially depend on how far you have to go to get your colostrum source. You can
buy commercial supplies of dehydrated packets that are freeze-dried colostrum. And they
usually list on the insert what they have antibodies to. The problem is that in the dairy
over in Sunnyside that I am working with, they use a mix. Say a dehydrated batch with say
with heifer’s colostrum if they can’t get a mature milk and colostrum in there.
So you can kind of mix, but that is expensive.