Immunocytes & Immunomodulators

Immunocytes & Immunomodulators

 PKGhatak,MD

Newton's first law of motion states that every action has an equal and opposite reaction. Our body is also governed by a similar law, for every function, there are the Stimulators and Suppressors mediated through proteins. Advances in molecular biology have led to the unlocking of secrets of the mechanisms by which the body detects and eliminates disease causing agents and cancer. This branch of therapeutics is called Immunotherapy and the agents are known as Immunomodulators.

Immunocytes produce proteins which act like keys and fit perfectly to a specific receptor present on the surface of the effector cells and initiate reactions and the response can be either stimulation or suppression of immune reactions. The immunocytes communicate constantly with each other and react in response to secreted proteins.

In the end, the Immunoglobulins are produced by Plasma cells. Immunoglobulins neutralize foreign agents and the natural killer cells (NK) and phagocytic cells engulf foreign substances and remove accumulated debris.

The immunoglobulins are many; written as IgM, IgG, IgA, IgD and IgE. The IgG is subdivided into many fractions. Both IgM and IgG are present in plasma, IgA is most abundant in the enterocytes of GI tract, genitourinary tract and in tear. IgD is bound to lymphocytes B (B cells). IgE appears in plasma, it is active against multicellular foreign organisms and parasites and is associated with allergy.

Immunocytes:

A group of cells - the White Blood Cells (WBC), Macrophages, Plasma cells and Dendritic cells are collectively called Immunocytes.

B-lymphocytes (B-cells)

Among the WBCs the Lymphocytes play a crucial role. Lymphocytes are divided into B-lymphocytes (B stands for bursa Fabricius of birds) and T-lymphocytes (T for Thymus) according to their origin. Both B-lymphocytes and T-lymphocytes are subdivided again into many numerical numbers according to the presence of surface CD (cluster designation) and are grouped as stimulators and suppressors.

During the maturation process, the B-lymphocyte travels to the thymus, spleen and lymph nodes. B-lymphocytes are activated by dendritic cells by presenting them with an antigen and the activated B-cells in lymph nodes act as temporary memory cells and the rest B-cells return to the bone marrow to become permanent Memory cells.

T-lymphocytes (T-cells).
T-lymphocytes originated in the embryonic Thymus gland. A subgroup of T-cells is natural killer cells (NK cells). T-lymphocytes perform a multitude of immunological functions along with B-cells.

Dendritic cells.

Dendritic cells are known as antigen presenting cells. In the skin, they are called Langerhans cells. In addition to skin, these cells are present in the nose, respiratory tract, stomach and intestine. Immature dendritic cells are present in peripheral blood. Dendritic cells attack foreign antigens and also diseased body cells and take a bite out of them and then process the antigen and finally hand over the antigen to plasma cells for antibody production. Also, the dendritic cells supply B-cells with antigens, and B-cells become memory cells.

Plasma cells.

Plasma cells originate in the spleen and lymph nodes from activated B-cells. Then the activated B-cells move to the bone marrow and reside there permanently. Plasma cells have large cytoplasm and an eccentrically located nucleus with coarse chromatin. Plasma cells are the chief producers of Immunoglobulins. Initial production of immunoglobulin is IgM specific to an antigen, then after a week or so the IgM production slows down and the plasma cells start to produce IgG antibodies in response to the same antigen.

Macrophages (large eaters).

Macrophages are derived from monocytes of blood. This transformation takes place as the monocytes extravasate the blood vessels in response to cytokines. Macrophages present in all tissues and assume different shapes are called histiocytes in connective tissue, Kupffer cells in the liver, osteoclast in bone, microglia cells in the brain, etc. Macrophages exhibit amoeboid movement. Macrophages encircle foreign substances like bacteria or virus particles or decaying cells and digest them and help clear the field of cellular debris prerequisite for repair.

Eosinophils.

Eosinophils originate in the bone marrow and are present in blood and all places except in the skin, lungs and esophagus. Eosinophils respond to multicellular organisms and parasites. Eosinophils are involved in anaphylaxis, asthma and atopic dermatitis, hay fever, and also produce many cytokines.

Basophils. Basophils produce heparin, histamine, serotonin, and IL-D4. Basophils have IgE receptors on the cell surface. After binding with the antigen, the conjugates initiate allergy and chronic inflammation to parasites. Like eosinophils, basophils are responsible for anaphylaxis, allergy and hay fever.
  

Cytokines:

These are products of immunocytes, cytokines are also produced by some non-immunocytes, like endothelial cells, fibroblasts and stromal cells. There are several cytokines and are named according to the substrate they act on, e.g., Interferons, Interleukins (ILs), Lymphokines, and TNF (tissues necrosis factor). Cytokines are peptides, secreted in minute amounts, measured in picograms (1pico= 0.001 nano) are very potent but the range is limited to cells of origin or nearby cells only. Cytokines may be inactivated by small molecules which form a covalent bond with the active site of cytokine and are known by a suffix "tinib" e.g.- Acalabrutinib.  Cytokine must attach to its specific receptors on the surface of effector cells and requires a perfect fit as a key fit with the lock in order to initiate the production of an enzyme (- kinase). Because cytokines are antigenic, specific antibodies can be produced in animals or in cell cultures against cytokines and cytokine receptors for therapeutic use.

Monoclonal antibodies (mAbs).

mAbs are produced by the activated plasma cells directed only to that specific antigen. The antigens may be a natural biological substances like viruses and bacterium, cytokines, cell surface receptors and also a lab engineered hybrid antigen. Cell culture is the preferred choice for manufacturing mAbs but suitable lab animals may be used also. Medical conditions where mAbs are in use are expanding, particularly in the treatment of malignancy. Some of the well-known conditions of mAbs use are malignant melanoma, rheumatoid arthritis, Crohn's disease, and multiple sclerosis. The currently available mAbs are – for TNF- Infliximab, etanercept, adalimumab, golimumab. For T-cell inhibition - abatacept. For B-cell inhibition - rituximab, belimumab. For IL-1 receptor inhibitor - anakinra. IL-6 receptor inhibitor- tocilizumab, sarilumab. For IL-6 cytokine – siltuximab.

Checkpoint inhibition.

Cell population at any moment is a balance between new cell formation and programmed cell deaths. Cell deaths are performed by an enzyme called Caspase. Cell surfaces have receptors for stimulators and inhibitors of this enzyme. NK cells carry activating ligands whereas Dendritic cells carry inhibitor ligands.

PD-1/PD-L1 monoclonal antibodies.

T-Cells express programmed cell death receptors on their surface. Dendritic Cells carry the PD-L1 ligand (PD-L1). When they bind, the conjugate acts as an “off-switch” and T-cell turns docile. It is known as checkpoint inhibition.

Cancer cells of the breast, stomach and other organs express PD-L1 legends on their surface. T-cells are attracted to cancer cells and bind with cancer cells, thereby making T-cells inactive. And thus, cancer cells escape NK cells and continue to grow.

Monoclonal antibodies PD-1/PD-L1 are Atezolizumab, Avelumab, Nivolumab and pembrolizumab. 

CART Therapy:

Surveillance T-cells have surface receptors by which T-cells attach to a foreign antigen. Because cancer cell antigen closely resembles the normal cell of the body, at times the receptors fail to bind with the cancer antigen. Mutated cancer cells have acquired this strategy and evade detection and death. In the CART therapy laboratory, Chimeric Antigen Receptors were engineered and harvested T-cells from the patient are incubated together. These special T-cells are made to multiply in the laboratory and later transfused to patients to enhance tumor killing.

EGF (Epidermal Growth Factor) and HER2 Receptor (human epidermal growth factor receptor2).

Epidermal Growth Factor (EGF) is a protein that binds with EGF Receptors on the cell surface initiates cell proliferation, and differentiation and prolongs cell survival. It works through the Tyrosine-kinase system.
Antibodies to EGF.
Currently, available mAbs that bind to EGF are Gefitinib, Erlotinib, and Afatinib.

Antibodies to HER2.

Cancer of some breasts, stomach and other solid organs is treated with the HER2 mAbs, e.g., Trastuzumab and Pertuzumab,

VEGRF.

The vascular epidermal growth factors can be blocked by antibodies, e.g.-Bevacizumab and ranibizumab

VEGRFRmAbs (Vascular epidermal growth factor receptor monoclonal antibodies.) Ramucirumab is available.

Polyclonal Antibodies (pAbs).

There are two ways to produce pAbs. One is from donated blood pools, isolating and purifying the antibodies, and the second method is to inject multiple antigens into a suitable animal and collect the antibodies. Animal origin pAbs are used in the treatment of snake bites, jellyfish toxins, spider bites, etc.

The pAbs are the mainstay of treatment of the immune thrombocytopenic purpura (ITP). RhD negative mothers, pregnant with a second child tested positive for the RhD, are treated with pAbs to prevent hydrops foetalis in newborns.

The pAbs are also produced in labs. Human T-cells are injected in horses or any other suitable lab animals. After several days blood is collected and the antibody containing immunoglobulins is purified. The pAbs  so engineered are mainly used in acute rejections following kidney transplants. It is also a useful way to treat diseases of uncertain etiology where runway immune reactions threaten life, as is happening in covid-19. Previously such pAbs were used in Ebola. MRSA infections. It is useful in Digitalis toxicity, Kawasaki disease and recent incidents of covid-19 Kawasaki syndrome. Guillain-Barre syndrome is treated with mAbs along with plasmapheresis. Similarly, Myasthenic crisis responds to a similar intervention.

Disease Modifying Drugs (DMD):

Human immunity has two components - the inborn or Innate immune system. Innate immune response to a foreign invading agent takes place within hours.  Adaptive Immune system response usually takes weeks to months. This adaptive system consists of 3 parts-namely antibodies, B-cells and T-cells.

Autoimmune diseases are treated with drugs that are in use for a long time. Most of the drugs are well known - like prednisone, cyclophosphamide, cyclosporine, methotrexate, azathioprine, gold compounds, sulfadiazine, etc.

Biological drugs are at present favored over traditional chemical compounds, though used together results are much better.

Some common uses of MAbs:

Rheumatoid arthritis. Overactive TNF is controlled by Etanercept containing a fusion protein IgG that binds with TNF alpha. Infliximab is a chimeric mAbs used for the same purpose. Adalimumab is a humanized mAbs also binds with TNF alpha.

IL-6(interleukin-6), an IL-6 receptor blocker, Tocilizumab, is used in solid organ rejection.
IL-2 receptor blockers are used in metastatic melanoma and renal cell carcinoma.

Psoriasis and Psoriatic arthritis.  Anti TNF therapy with mAbs in psoriasis and psoriatic arthritis responds well.

Polymyositis and dermatomyositis. B-cell mAbs, Rituximab, is used with success.

IgG4 related disease has a varied presentation. Rituximab with conventional DMD are used. 

Wegener Granulomatosis / Granulomatosis with polyangiitis. It is associated with antineutrophil cytoplasmic antibodies (ANCA). Anti B-cell mAbs, rituximab, with prednisone are commonly used.
In asthma Omalizumab, a IgE inhibitor is used.

Antiviral mAbs. 
Bavituximab, a serine mAb used in Hepatitis C.
Palivizumab, a RSV virus mAb is used in respiratory syncytial virus bronchiolitis and pneumonia.
Anakinra and Tocilizumab, IL-1 and IL-6 receptor inhibitors respectively, are at present undergoing trials in COVID-19 pneumonia and multi organ failure.

Use of mAbs in hematologic cancers
Rituximab, a CD B20 chimeric mAb used in non-Hodgkin's lymphoma.
Alemtuzumab, a CD52 on B-cell & T-cells used in B cell leukemia. 
Gemtuzumab, a myeloid cell antigen CD33 humanized mAb used in relapsed myeloid leukemia.

Recently mAbs are approved for the treatment of neurological diseases. 

Multiple Sclerosis (MS). MS is an autoimmune disease resulting in damage to myelin sheath from cytokines overproduction. Glatiramer acetate binds with CD4B-cells and CD4T cells and decreases cytokines production.

Natalizumab, an alpha 4 integrin mAb, binds with immunocytes and prevents them from crossing the blood brain barrier.
Rituximab, an anti CD20 mAb lowers complement and cytokines.
Ocrelizumab acts in the same manner.
Alemtuzumab binds with TCD52. Fingolimod is an analog to sphingosine. It binds with sphingosine receptors on T-cells and B-cells and limits cytokine production.

Migraine.
The etiology of migraine is not fully understood but migraine pain is due to the release, at the end of the nerve terminals, a preformed calcitonin gene related protein (CGRP) at the junction of nerve terminals and smooth muscles in the intracranial vessels. CGRP protein binds with CGRP receptors (CGRPR) present on the blood vessels and results in a release of pain causing substance P. Examples are -  Erenumab.
Frenanezumab and Galcanezumab are mAbs neutralize CGRP ligand and galcanezumab use limits the incidence of migraine attacks and shortens duration of migraine.

Adverse effects:
 Disease modifying drugs are an effective treatment for autoimmune diseases and cancers. Biologic modifying agents are used in increasing numbers. However, biological substances have serious side effects. Infection with atypical organisms, atypical mycobacteria, fungal infections and the recurrence of previous viral illnesses - particularly Hepatitis B and C and cytomegalovirus are problematic. Reactivation of old TB and varicella are not unusual. In long term follow up of biological DMD shows an increased incidence of lymphoma and leukemia.

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