Newly differentiated blood cells continually enter the bloodstream from their sites of production: red blood cells, monocytes, granulocytes, and B lymphocytes from the bone marrow: and T lymphocytes from the thymus. Under ordinary circumstances red blood cells spend their entire life span of 120 days in the bloodstream. However, white blood cells are destined to emigrate from the blood to perform their effector functions. Lymphocytes recirculate through secondary lymphoid tissues, where they are detained if they encounter an antigen to which they can respond; macrophages migrate into the tissues as they mature from circulating monocytes; effector T lymphocytes and large numbers of granulocytes are recruited to extravascular sites in response to infection or injury. For example, it is estimated that, each day, three billion neutrophils enter the oral cavity, the most contaminated site in our body.

The process by which white blood cells migrate from the bloodstream to sites of infection is fairly well understood. First their flow is retarded by the interaction between selectins whose expression is induced on activated vascular endothelium and certain fucosylated glycoprotiens on the white cell surface (for example sialyl-Lewis x ). Tight binding of leukocytes to the endothelial surface is then triggered by chemokines, such as interleukin-8, which activate an enhanced ability of the leukocyte integrins (for example LFA-1 and Mac-1) to adhere to their receptors. Crossing the endothelial cell wall also involves interactions between the leukcyte integrins and their receptors, while the subsequent direction of migration follows a concentration gradient of chemokines (for example IL-8) produced by cells already at the site of infection or injury. The process by which lymphocytes home to secondary lymphoid tissue is very similar, except that it is initiated by mucinlike addressins on lymphoid venules binding to L-selectin on the surface of naive lymphocytes.

An excellent opportunity to study the role of integrins is provided by a genetic defect in CD18, the common ß chain of the three ß2 integrins: LFA-1 (CD11a:CD18), Mac-1 (CD11b:CD18, also known as CR3), and p150,95 (CD11c:CD18, also known as CR4). Children with this genetic defect suffer from leukocyte adhesion deficiency. They have recurrent pyogenic infections, problems with wound healing, and if they survive long enough they develop severe inflammation of the gums (gingivitis). Surprisingly, children with leukocyte adhesion deficiency are not unduly susceptible to opportunistic infections. This implies normal T-cell function despite the absence of LFA-1, which was thought of to be important for T-cell adhesion to antigen-presenting cells. The capacity to form antibodies is also unimpaired, showing that adequate collaboration between T and B cells can also occur without LFA-1.

The case of Luisa Ortega: a problem of immobile white blood cells.

Luisa Ortega was born at full term and weighed 3.7 kg. She was the second child born to the Ortegas. At 4 weeks of age Luisa was taken by her parents to her pediatrician because she had swelling and redness around the umbilical cord stump (omphalitis), and a fever of 39 o C. Her white blood cell count was 71,000µl- 1 (normal 5,000-10,000 µl- 1 ). She was treated in the hospital with intravenous antibiotics for 12 days and then discharged home with oral antibiotics. At the time of discharge her white blood count was 20,000 µl- 1 . Cultures obtained from the inflamed skin about the umbilical stump before antibiotic treatment grew Escherichia coli and Staphylococcus aureus .

The Ortegas had a baby boy 3 years prior to Luisa's birth. At 2 weeks of age he developed a very severe infection of the large intestine (necrotizing enterocolitis). Separation of his umbilical cord was delayed. He subsequently suffered from multiple skin infections and he died of staphylococcal pneumonia at 1 year of age. Just before his death his white blood cell count was recorded at 75,000 µl- 1 .

Because of the previous family history, Luisa was referred to the Children's Hospital. At the time of her admission to the Children's Hospital she seemed normal on physical examination, and radiographs of the chest and abdomen were normal.

Cultures of urine, blood and cerebrospinal fluid were negative. Her white blood count was 68,000 µl- 1 (very elevated). Of her white cells, 73% were neutrophils, 22% lymphoctes, and 5% eosinophils (this distribution of cell types is in the normal range but the absolute count for each is abnormally high). Her serum IgG concentration was 613 mg dl- 1 (normal), her IgM was 89 mg dl- 1 (normal), and her IgA 7 mg dl- 1 (normal). The concentration of complement component C3 in her serum was 185 mg dl- 1 and that of C4 was 28 mg dl- 1 (both normal).

A Rebuck skin window was performed. In this procedure, the skin of the forearm is gently abraded with a scalpel blade and a cover slip is placed on the abrasion. After 2 hours the cover slip is removed and replaced by another every subsequent 2 hours for a total of 8 hours. In this way, the migration of immune cells into the damaged skin can be monitored. No white cells accumulated on the cover slips. All of Luisa's blood leukocytes, however, were present in abnormally high numbers. Of her blood lymphocytes 53% (7930 µl- 1 ) were T cells (as measured by CD3 expression); of these, 36% were CD4 and 16% CD8 (normal proportions); 25% (3754 µl- 1 ) were B cells (as measured by antibody to CD19), and 14% were NK cells (as measured by CD16 expression). These were both elevated.

Proliferation of Luisa's T cells in response to phytohemagglutinin (PHA) and concanavalin A (Con A) was slightly depressed. Further flow cytometric analysis revealed that, whereas 60% of Luisa's lymphocytes were stained by a monoclonal antibody to CD3, only 5% reacted with a monoclonal antibody to CD18, giving a CD18/CD3 ratio of 5%/6% compared with 62%/65% on testing cells from a control subject. Her blood mononuclear cells were stimulated with PHA and examined after 3 days of incubation with monoclonal antibody to CD11a (the a chain of LFA-1). No LFA-1 expression was found.

Luisa was treated with busulfan, cyclophosphamide and anti-thymocyte serum for 10 days. After this therapy, she was given bone marrow cells from her mother at a dose of 500 X 10 6 per kg body weight, and a short course of immunosuppressive therapy. Her mother's bone marrow donation had been depleted of mature T cells with a monoclonal antibody to mature T cells and complement. Twenty-eight days after the transplant, the lymphoid and myeloid cells in Luisa exhibited complete chimerism. She subsequently did well clinically and her white blood count remained at 7800 µl- 1 .

Luekocyte adhesion deficiency

Children like Luisa are subject to recurrent, severe bacterial infections that are eventually fatal. In these patients, encapsulated bacteria are coated with antibody and complement. However, the neutrophils and monoctyes that would normally be recruited to the site of infection are entrapped in the blood stream and cannot emigrate into the tissues because they lack LFA-1 (CD11a:CD18) and Mac-1/CR3 (CD11b:CD18). In a normal individual, the first cover slip in a Rebuck glass window would contain many neutrophils. Monocytes begin to appear at 4 hours and by 8 hours the cover slip contains predominantly monocytic cells and very few neutrophils. In Luisa's case the cover slips had no cells because her leukocytes were unable to emigrate from the bloodstream and onto the cover slip. For this reason, the white cells in the bloodstream are very high: a very high white blood cell count is characteristic of leukocyte adhesion deficiency. The ability to deal with pyogenic bacteria is further compromised because of the vital role of CR3-mediated uptake of these opsonized bacteria by neutrophils. The role of CD4 or CD11c:CD18, the third member of the ß2 integrin family, is less well understood but, like CR3, it binds complement fragments, and it is thought to have a role in uptake of bacteria by macrophages.

The importance of the daily, massive neutrophil emigration into the oral cavity is well illustrated by individuals with leukoctye adhesion deficiency, who invariably develop severe gingivitis when they survive. Another, poorly understood, consequence of the lack of leukocyte emigration is the failure to heal wounds. Delayed separation of the umbilical cord is the earliest manifestation of this defect in wound healing. Subsequently, affected children may develop fistulas (abnormal connecting channels) in their intestine after bacterial infections of the gut.

Bone marrow transplantation has been very successful in rescuing severely affected infants from certain death.

We would like to thank Dr. Fred Rosen and Dr. Raif Geha for their contribution of the above information from their book, "Case Studies in Immunology 3."

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