Treatments to Modulate the Host Immune System

Question 1

exogenous immune suppression two pathways

Answer 1

1. Using chemicals or agents that
   are broad or non-specific in that they can also suppress the activity of other cell types in the
   body.
2. specific in which the target is a specific immune cell or mediator. We will first review the chemicals and agents that can be used to non- specifically suppress the immune system.

Question 2

Non-specific Immunosuppressants:

Answer 2

corticosteroids, cytotoxic drugs, radiation tx

Question 3

what are corticosteroids

Answer 3

This group of drugs are the most commonly and widely used
immunosuppressive and anti-inflammatory drugs in human and veterinary medicine.
Interestingly, their level of efficacy differs by species. For example, humans and rodents are
more corticosteroid sensitive than the domestic veterinary species. The important point is that the
type and dose of corticosteroid use will be dictated by the species receiving the drug.
Corticosteroids are classified as either glucocorticoids (i.e. anti-inflammatory) or
mineralocorticoids (i.e. salt retainers). As glucocorticoids, they suppress inflammation and
immunity, while also modulating the breakdown of carbohydrates, fats and proteins. As
mineralocorticoids, they can regulate the salt and water balance in the body. One last point,
corticosteroids can have different effects on different cells in the body

Question 4

mode of action of corticosteroids

Answer 4

The drug, due to it lipophilic properties, can easily gain entry in to the host cells
and bind to cytoplasmic corticosteroid receptors. These complexes then enter the nucleus where
alters protein synthesis. Depending on the cell type, it can affect the protein that is synthesized
and thus, the function of the cell (i.e. metabolic function of cells, cell membrane permeability. In
immune cells, it can increase the production of the protein IκBα which is typically bound to the
transcription factor NF-κB in the cytoplasm of resting immune cells. Normally when immune
cells become activated, IκBα is cleaved and digested by proteasomes in the cytoplasm and NF-κB is trafficked into the nucleus where initiates protein synthesis of potent proinflammatory
cytokines. Corticosteroids block this protein synthesis by preventing the trafficking of NF-κB
into the nucleus by providing more IκBα to rebind to the NF-κB. Corticosteroids can also inhibit
phospholipase A, which is important in arachidonic acid synthesis, an important molecule
involved in the formation of potent inflammatory mediators

Question 5

4 areas associated with general effects of corticosteroids

Answer 5

Corticosteroids can alter the metabolism of carbohydrates, fat and proteins. As described
above, they alter the activities of inflammatory mediators. They can modulate leukocyte
production and circulation (i.e. decreased chemotaxis and phagocytosis). Lastly, they can alter
the effector function of lymphocytes (i.e. decreased proliferation, IL-2 production T cell
function)

Question 6

risks associated with corticosteroids

Answer 6

Long term administration of corticosteroids can
suppress the adrenal-pituitary axis leading to a disease known as Cushing’s syndrome

Question 7

the common synthetic corticosteroids administered in humans

Answer 7

prednisone, prednisolone,
methylprednisolone and dexamethasone

Question 8

the common synthetic corticosteroids administered in companion animals

Answer 8

prednisolone and
methylprednisolone

Question 9

the common synthetic corticosteroids administered in large animals

Answer 9

betamethasone and dexamethasone

Question 10

what are targeted treatments of corticosteroids

Answer 10

allergies, arthritis, asthma, skin rashes and colitis

Question 11

list of cytotoxic drugs

Answer 11

alkylating drugs, folic acid antagonists, DNA synthesis inhibitors

Question 12

alkylating drugs

Answer 12

cytotoxic drugs that broadly work, mode of action, by
adding an alkylating agent to a guanine base on DNA in actively dividing cells. This
results in cross-linking DNA helices resulting in DNA strand breaks, preventing cell
division leading to the death of the cell. These were one of the first class of drugs to
treat cancer (i.e. antineoplastic or anti-cancer drug) as early as the 1940’s and are
classified as a chemotherapeutic drug. There are 5 types of alkylating agents: nitrogen
mustards, nitrosoureas, alkyl sulfonates, triazines and ethylenimines.
Cyclophosphamide (cyclophosphane, cytoxan), a nitrogen mustard, is one of the most
well-known and commonly used of the alkylating agents and requires metabolism in the
liver in order to become active. It has tremendous suppressive effects on T cell and B cell
function as well as being myelosuppressive

Question 13

Cyclophosphamide

Answer 13

in addition to being immunosuppressive, has been used to treat
lymphoma, multiple myeloma, breast cancer, neuroblastoma, ovarian cancer, small cell
lung cancer and sarcoma

Question 14

Folic Acid Antagonists

Answer 14

class of drugs that block cells from using folic acid to
synthesize DNA constructs making it also an effective anti-neoplastic drug.
Methotrexate is classic folic acid antagonist that binds to dihydrofolate reductase
preventing the synthesis of tetrahydrofolate, which blocks the synthesis of thymidine
and purine nucleotides

Question 15

Methotrexate

Answer 15

can suppress antibody formation and as a result also has good efficacy
against rheumatoid arthritis and severe psoriasis

Question 16

DNA Synthesis Inhibitors

Answer 16

similar to folic acid inhibitors, DNA synthesis inhibitors block
DNA synthesis. Azathioprine (Imuran) functions as a nucleoside analog to block DNA
synthesis. It is converted in the liver to 6-mercaptopurine via thiopurine
methyltransferase resulting in the inhibition of both RNA and DNA synthesis

Question 17

Azathioprine

Answer 17

can suppress both primary and secondary antibody responses as well as T
cell-mediated response and macrophage production. It is also myelosuppressive. It is
used to treat cancer, dermatomyositis, inflammatory bowel disease, rheumatoid arthritis,
lupus (SLE), vasculitis and commonly partnered with corticosteroids.

Question 18

Radiation treatment

Answer 18

Radiation exists as two different forms, non-ionizing and ionizing.
In general, the non-ionizing form of radiation tends to be safe. It is the ionizing
form that tends to be the more detrimental to the cells and/or host. Ionizing radiation emits as
high energy, that upon entry into the cell, can directly damage DNA as well as ionizes water
molecules forming free radicals in the cells. Both forms of cell damage can result in cell death.
Radiation therapy can originate from an external source (i.e. gamma radiation accelerator) or
placed inside the body known as brachytherapy using radiative beads, one treatment for prostate
cancer
In the past, it has been used to suppress immunity, in particular T cells, in graft transplants for
research animals, it is not a practical tool in human or veterinary medicine

Question 19

Specific or Selective Immunosuppressants

Answer 19

Calcineurin Inhibitors (CNI) (cyclosporine), mTOR (Mechanistic Target of Rapamycin) Inhibitors (rapamycin), Inosine Monophosphate Dehydrogenase Inhibitors (Mycophenolate mofetil), Leflunomide (Arava), Intravenous Immunoglobulin Therapy (IVIG)

Question 20

Calcineurin Inhibitors (CNI)

Answer 20

family of drugs (cyclosporine, pimecrolimus, and
tacrolimus) with potent selective immunosuppressive properties. Broadly speaking, their mode of
action is to block T cell proliferation via the inhibition of the key signaling phosphatase
calcineurin. Cyclosporine binds to the intracellular protein cyclophilin. Tacrolimus binds to FK-
binding protein and pimeocrolimus binds to macrophilin-12. When these complexes are formed
within the cell, calcineurin is inhibited and T cell activation is blocked

Question 21

Cyclosporine

Answer 21

most well-known and utilized polypeptide product of
the soil fungus, Tolypocladium inflatum, which naturally yields several forms of the peptide.
Cyclosporin A is a circular 11 amino acid with 2 protein binding sites. Upon entry into T cells, it
binds to cyclophilin at one site and calcineurin, a serine threonine phosphatase, with the other
site. This effectively shutdowns down signal transduction inhibiting IL-2 and IFNγ resulting in
the downregulation of Th1 response. Along with corticosteroids, this drug combination is quite
effective in controlling against immune-mediated graft rejections.

Question 22

mTOR (Mechanistic Target of Rapamycin) Inhibitors

Answer 22

Mechanistic target of rapamycin
(mTOR) is a key nutrient-sensitive regulator of cell growth in animals with a major role in cell
physiology, metabolism, aging and when upregulated, can lead to disease such as cancer. It
functions as the catalytic subunit for two protein complexes, mTORC1 and mTORC2.
Downregulation or inhibition of mTORC1 is associated with increased lifespan in animal models
(i.e. rodents)

Question 23

rapamycin

Answer 23

an inhibitor of mTOR, is a natural anti-fungal antibiotic of soil bacteria of
the Eastern Island. quite effective in regulating T cell activation, inhibiting T cell and B cell
proliferation and can promote tolerance. It is often paired with a calcineurin inhibitor to prevent
graft rejection. Recently, this drug and its synthetic analogs or rapalogs has received a lot of
attention as a possible treatment to extend the quality and length of life. Based on its mode of
action, it does make some sense although more studies are needed in this area

Question 24

Inosine Monophosphate Dehydrogenase Inhibitors:

Answer 24

Inosine Monophosphate Dehydrogenase
is responsible for the rate limiting step in the de novo synthesis of guanine nucleotides. Guanine
nucleotide synthesis is critical for normal cell function and growth as well as the immune
response

Question 25

Mycophenolate mofetil

Answer 25

an Inosine Monophosphate Dehydrogenase inhibitor, is an anti-
bacterial fungus used as a immunosuppressant to control organ transplant (i.e. kidney, heart,
liver) rejection and immune-mediated disease (i.e. Crohn’s disease, SLE, ITP, pemphigus
vulgaris). It is reported to block dendritic maturation, T cell and B cell proliferation, T cell
differentiation, and antibody production. It is often paired with other immunosuppressive drugs

Question 26

Leflunomide (Arava)

Answer 26

classified as a disease-modifying antirheumatic drug (DMARD). It
blocks pyrimidine synthesis and thus, prevents DNA replication in cells (i.e. cells of the immune
system). It is sometimes used to block graft rejection in companion animals. It has been used
quite extensively as a treatment for human rheumatoid arthritis

Question 27

intravenous Immunoglobulin Therapy (IVIG)

Answer 27

species-specific blood product that is
prepared from the serum of many donors (i.e. > 1000 donors per batch). The primary purified
components are antibodies (>95%) so you may see IVIG therapy also called gamma-globulin
therapy. It was first used in the early 1950’s to treat primary immune deficiency. In humans, it
has been used to treat primary immune deficiencies and immune-mediated thrombocytopenia
purpura (ITP). As an immunomodulating agent, IVIG can modulate host complement activation,
suppress idiotypic antibodies, saturate Fc receptors on macrophages as well as suppress
cytokines, chemokines and metalloproteases. All of the above events can help neutralize select
host immune-mediated diseases (i.e. Kawasaki disease, acquired hemophilia). IVIG also contains
natural antibodies and cytokines that can add some level of protection to the recipient host

Question 28

immunostimulants

Answer 28

Bacteria and Bacterial byproducts, Complex carbohydrates, Cytokines and monoclonal antibodies, vitamins

Question 29

Bacteria and Bacterial byproducts

Answer 29

Bacterial components have been used as
immunomodulators for >40 years. The initial target is the innate immune system via pathogen-
associated molecular patterns to stimulate TLRs. This leads to the activation of macrophages and
dendritic cells with their cadre of proinflammatory cytokines. Bacillus Calmette-Guérin (BCG) is
one of the oldest and well-known bacterial products used in medicine. It is a modified-live strain
of Mycobacterium bovis. In fact, at the correct formulation, it is used as a vaccine in humans
against Mycobacterium tuberculosis (TB). Typically, just purified cell wall components of the
bacteria are used as an immunostimulant. In horses, BCG has been successfully used to treat
equine sarcoid tumors and ocular squamous cell carcinoma. In humans, BCG is used to treat
early stage bladder cancer and melanoma.

Question 30

complex carbohydrates

Answer 30

These are immunostimulants derived from yeasts. An example
includes zymosan, particles of yeast cell walls, that is a glucan with repeating glucose units
bound by b-1,3-glycosidic linkages. It is prepared from baker’s yeast (Saccharomyces cerevisiae
). Zymosan particles are rapidly ingested by phagocytic cells resulting in activation macrophage
activation of TLR2 and NF-B) and the release of inflammatory cytokines. This product has
been used quite effectively in aquaculture facilities mixed in with feed

Question 31

Cytokines and monoclonal antibodies

Answer 31

When the discovery and our understanding of the
function of cytokines began to grow, individuals in the medical and research community initially
thought that cytokines could be a good form of therapy. Thus, cytokines were administered to
augment and boost immunity in folks suffering from a host of illnesses. Unfortunately, this
proved to yield unpleasant outcomes with people and animals having adverse response to the
cytokines. It turns out that the cytokines could affect multiple cells in the body not just the
intended target. As a result, cytokine therapy is not employed in veterinary medicine and only
sparingly in human medicine and with only select cytokines. For example, IL-2 cytokine therapy
has been used successfully to treat patients with metastatic renal cancer and melanoma. In
immunocompromised patients (i.e. HIV or aged), IL-7 has proven successful in regenerating T
cells. Further, IL-15 therapy promoted the growth and function of T and NK cells in cancer
patients. In summary, select cytokine therapy does have some application in individuals that are
immunocompromised or suffering from neoplasia.
If you recall when I talked about antibodies during an earlier session, I indicated that this
was one the most important group of proteins in medicine in that it was analyzed to assess host
immune health status and response to vaccines; it was used in many diagnostic assays; and it
could be used as a treatment. This treatment dates back at least to the 1900s when horse serum
(gamma globulins) were used to treat soldiers during WW I suffering from serious bacterial
infections. Eventually, antibiotics (i.e. penicillin) were discovered and replaced this practice.
Presently, antibodies are used to combat cancer, infections and many other diseases. Due to our
technological advances, we have been able to not only produce in a pure form using a single
clone (monoclonal), but we have been able to genetically engineer these antibodies to have
antigenicity to the host and be multi-functional resulting in a > 300 billion-dollar industry. I say
this to emphasize how effective this form medicine has become. Currently in 2020, there are at
least 78 commercial antibodies available as a therapeutic treatment. I am just going to highlight a
few of them. Interesting enough, the names of all these commercial monoclonal antibodies end
with the letters “mab”. Rituximab (trade name Rituxan) is a chimeric IgG1, approved in 1997,
against CD20, a cell surface marker/protein on B cells, to treat Non-Hodgkin lymphoma.
Omaliizumab (trade name: Xolair) is a humanized IgG1, approved in 2003, against IgE to treat
patients with asthma. A more recent monoclonal antibody is Dupilumab (trade name: Dupixent),
which is a humanized IgG4, approved in 2017, against IL-4R α to treat patients with atopic
dermatitis. All these monoclonal therapies have shown great promise and more are likely on the
way

Question 32

vitamins

Answer 32

Vitamins have also been reported to have a certain degree of immunomodulating
capability. It is important note that the type of vitamin intake to boost immunity greatly depends
on the species. In humans, there is significant debate on what are the best vitamin supplements to
boost human health and this is even more important given the COVID crisis. Not surprisingly,
eating a diet high in fruits in vegetables goes a long way to supporting and enhancing immune
health. If such a diet is not achievable, then by taking supplements containing vitamin C, vitamin
D and zinc is a good start. In animals, supplements containing vitamins A, D and E plus
selenium have been shown to enhance immune health. Vitamin A supplementation has been
shown enhance T cell cytotoxicity and proliferation. Vitamin D enhances macrophage and
dendritic cell function and vitamin E with selenium can promote B cell function and
proliferation. Further, Vitamin E supplementation is believed to enhance immune function in the
elder population including humans