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Severe combined immunodeficiency

Severe Combined Immunodeficiency, or SCID, is a genetic disorder in which both "arms" (B cells and T cells) of the adaptive immune system are crippled, due to a defect in one of several possible genes. SCID is a severe form of heritable immunodeficiency. It is also known as the "bubble boy" disease because its victims are extremely vulnerable to infectious diseases and must live (if untreated) in a completely sterile environment. The most famous case is the boy David Vetter. more...

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SCID affects about 1 in 80,000 live births. These babies, if untreated, usually die within 1 year due to severe, recurrent infections. Chronic diarrhea, ear infections, recurrent Pneumocystis jiroveci pneumonia, and profuse oral candidiasis commonly occur. Treatment options are much improved since David Vetter, and living in a bubble is no longer necessary.

Types

IL-7 signalling pathway

Most cases of SCID are derived from mutations in the γc chain in the receptors for interleukins IL-2, IL-4, IL-7, IL-9 and IL-15. These interleukins and their receptors are involved in the development and differentiation of T and B cells. Deleterious mutations in the gamma-chain or in the JAK3 protein cause a form of SCID that is characterized by low numbers of T and NK cells, and presence of non-functional B cells.

The IL-2 receptor γ (IL-2Rγ) gene is located on the X chromosome and mutation of this gene causes X-linked SCID.

Janus kinase-3 (JAK3) is an enzyme that mediates transduction downstream of the γc signal. Mutation of its gene also causes SCID.

VDJ recombination

The manufacture of immunoglobulins requires recombinase enzymes derived from the recombination activating genes RAG-1 and RAG-2. These enzymes are involved in the first stage of VDJ recombination, the process by which segements of a B cell or T cell's DNA are rearranged to create a new T cell receptor or B cell receptor (and, in the B cell's case, the template for antibodies). Certain mutations of the RAG-1 or RAG-2 genes prevent VDJ recombination, causing SCID.

Adenosine deaminase

Another well-known form of SCID is caused by a defective enzyme, adenosine deaminase (ADA), necessary for the breakdown of purines. Lack of ADA causes accumulation of dGTP. This metabolite is toxic to lymphoid stem cells.

Detection

Standard testing of SCID is not performed for newborns due to the rarity of the disease and the cost of the testing. SCID can be detected by sequencing fetal DNA if a known history of the disease exists. Otherwise, SCID is not detected until about six months of age, usually indicated by recurrent infections. The delay in detection is due to the fact that newborns carry their mother's antibodies for the first few weeks of life and have not yet been exposed to any diseases.

Treatment

The most common treatment for SCID is bone marrow transplantation, which requires matched donors (a sibling is generally best). David Vetter, the original "bubble boy," endured several failed transplantations, and finally passed away because of an unscreened virus, Epstein-Barr, in his newly-transplanted bone marrow from his sister. Today, transplants done in the first three months of life have a high success rate.

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Pneumocystis pneumonia
From Gale Encyclopedia of Medicine, 4/6/01 by Cindy L. Jones

Definition

Pneumocystis pneumonia is a lung infection that occurs primarily in people with weakened immune systems-- especially people who are HIV-positive. The disease agent is an organism whose biological classification is still uncertain. Pneumocystis carinii was originally thought to be a one- celled organism (a protozoan), but more recent research suggests that it is a fungus. Although its life cycle is known to have three stages, its method of reproduction is not yet completely understood. The complete name of the disease is Pneumocystis carinii pneumonia, often shortened to PCP. PCP is also sometimes called pneumocystosis.

Description

Pneumonia as a general term refers to a severe lung inflammation. In pneumocystis pneumonia, this inflammation is caused by the growth of Pneumocystis carinii, a fungus-like organism that is widespread in the environment. PCP is ordinarily a rare disease, affecting only people with weakened immune systems. Many of these people are patients receiving drugs for organ transplants or cancer treatment. With the rising incidence of AIDS, however, PCP has become primarily associated with AIDS patients. In fact, as many as 75% of AIDS patients have developed PCP. It has also been the leading cause of death in AIDS patients.

Transmission

The organism that causes PCP is widely distributed in nature and is transmitted through the air. When the organism is inhaled, it enters the upper respiratory tract and infects the tiny air sacs at the ends of the smaller air tubes (bronchioles) in the lungs. These tiny air sacs are called alveoli. Under a microscope, alveoli look like groups of hollow spheres resembling grape clusters. The exchange of oxygen with the blood takes place in the alveoli. It appears that P. carinii lives in the fluid in the lining of the alveoli.

Person-to-person infection does not appear to be very common; however, clusters of PCP outbreaks in hospitals and groups of immunocompromised people indicate that patients with active PCP should not be exposed to others with weakened immune systems. It is thought that many people actually acquire mild Pneumocystis carinii infections from time to time, but are protected by their immune systems from developing a full-blown case of the disease.

Causes & symptoms

Causes

P. carinii is an opportunistic organism. This means that it causes disease only under certain conditions, as when a person is immunocompromised. Under these circumstances, P. carinii can multiply and cause pneumonia. The mechanisms of the organism's growth within the alveoli are not fully understood. As the pneumocystis organism continues to replicate, it gradually fills the alveoli. As the pneumonia becomes more severe, fluid accumulates and tissue scarring occurs. These changes result in decreased respiratory function and lower levels of oxygen in the blood.

High-risk groups

Some patients are at greater risk of developing PCP. These high-risk groups include:

  • Premature infants
  • Patients with immunodeficiency diseases, including severe combined immunodeficiency disease (SCID) and acquired immunodeficiency syndrome (AIDS).
  • Patients receiving immunosuppressive drugs, especially cortisone-like drugs (corticosteroids)
  • Patients suffering from protein malnutrition.

AIDS is currently the most common risk factor for PCP in the United States. PCP is, however, also found in countries with widespread hunger and poor hygiene.

Symptoms

The incubation period of PCP is not definitely known, but is thought to be between four and eight weeks. The major symptoms include shortness of breath, fever, and a nonproductive cough. Less common symptoms include production of sputum, blood in the sputum, difficulty breathing, and chest pain. Most patients will have symptoms for one to two weeks before seeing a physician. Occasionally, the disease will spread outside of the lung to other organs, including the lymph nodes, spleen, liver, or bone marrow.

Diagnosis

The diagnosis of PCP begins with a thorough physical examination and blood tests. Although imaging studies are helpful in identifying abnormal areas in the lungs, the diagnosis of PCP must be confirmed by microscopic identification of the organism in the lung. Samples may be taken from the patient's sputum, or may be obtained via bronchoscopy or lung biopsy. Because of the severity of the disease, many physicians will proceed to treat patients with symptoms of pneumocystis pneumonia if they belong to a high-risk group, without the formality of an actual diagnosis. The severity of PCP can be measured by x-ray studies and by determining the amount of oxygen and carbon dioxide present in the patient's blood.

Treatment

Treatment for PCP involves the use of antibiotics. These include trimethoprim-sulfamethoxazole (TMP-SMX, Bactrim, Septra) and pentamidine isoethionate (Nebupent, Pentam 300). Both of these anti-microbial drugs are equally effective. AIDS patients are typically treated for 21 days, whereas non-AIDS patients are treated for 14 days. TMP-SMX may be highly toxic in AIDS patients, causing severe side effects that include fever, rash, decreased numbers of white blood cells and platelets, and hepatitis. Pentamidine also causes side effects in immunocompromised patients. These side effects include decreased blood pressure, irregular heart beats, the accumulation of nitrogenous waste products in the blood (azotemia), and electrolyte imbalances. Pentamidine can be given in aerosol form to minimize side effects. Alternative drugs can be used for patients experiencing these side effects.

P. carinii appears to be developing resistance to TMP- SMX. In addition, some patients are allergic to the standard antibiotics given for PCP. As a result, other antibiotics for the treatment of PCP are continually under investigation. Some drugs proven to be effective against P. carinii include dapsone (DDS) with trimethoprim (Trimpex), clindamycin (Cleocin) with primaquine, as well as atovaquone (Mepron). Paradoxically, corticosteroids have been found to improve the ability of TMP-SMX or pentamidine to treat PCP. As a treatment of last resort, trimetrexate with leucovorin (Wellcovorin) can also be used.

Prognosis

If left untreated, PCP will cause breathing difficulties that will eventually cause death. The prognosis for this disease depends on the amount of damage to the patient's lungs prior to treatment. Prognosis is usually better at a facility that specializes in caring for AIDS patients. Antibiotic treatment of PCP is about 80% effective.

Prevention

Medications

For patients at serious risk for PCP infection, low doses of TMP-SMX, given daily or three times a week, are effective in preventing PCP. The drug is, however, highly toxic. Researchers are currently evaluating the effectiveness and toxicity of aerosol pentamidine and dapsone in preventing PCP.

Lifestyle modifications

Patients who have previously had PCP often experience a recurrence. Healthy lifestyle choices, including exercising, eating well, and giving up smoking may keep the disease at bay.

Key Terms

Alveoli
Small, hollow air sacs found in the lungs at the end of the smaller airways (bronchioles). Air exchange occurs in the alveoli.
Azotemia
The presence of excess nitrogenous wastes in the blood.
Biopsy
A procedure in which a piece of tissue is obtained for microscopic study.
Bronchoscopy
A procedure that uses a fiber-optic scope to view the airways in the lung.
Fungus
A single-celled form of plant life that lives on organic material, including human tissues.
Pentamidine isoethionate
An antibiotic used to treat and prevent PCP.
Pneumocystosis
Another name for active PCP infection.
Protozoan
A microorganism belonging to the Protista, which includes the simplest one-celled organisms.
Sputum
A substance obtained from the lungs and bronchial tubes by clearing the throat or coughing. Sputum can be tested for evidence of PCP infection.
Trimethoprim-sulfamethoxazole (TMP-SMX)
An antibiotic used to treat and prevent PCP.

Further Reading

For Your Information

    Books

  • Dobkin, Jay. "Pneumocystis Carinii Pneumonia (PCP)." In The Columbia University College of Physicians and Surgeons Complete Home Medical Guide. New York: Crown Publishers, Inc., 1995.
  • Walzer, Peter D. "Pneumocystis Carinii." In Principles and Practice of Infectious Diseases, edited by G. L. Mandell, et al. New York: Churchill Livingston, 1995.

    Periodicals

  • Korraa, H., and C. Saadeh. "Options in the Management of Pneumonia Caused by Pneumocystis Carinii in Patients with Acquired Immune Deficiency Syndrome and Intolerance to Trimethoprim/Sulfamethoxazole." Southern Medical Journal 89, no. 3 (March 1996): 272-277.

    Organizations

  • American Lung Association. (800)LUNG-USA. http://www.lungusa.org.
  • National Center for Infectious Disease, Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta GA 30333. http://www.cdc.gov.

Gale Encyclopedia of Medicine. Gale Research, 1999.

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