LAD type I (LAD I) is a rare AR disorder of leukocyte adhesion, chemotaxis, and ingestion of C3bi-opsonized microbes as a result of decreased or absent expression of the leukocyte β2 integrins (Table 1). The hallmark of LAD I is the occurrence of repeated, often severe bacterial and fungal infections without the accumulation of pus despite persistent granulocytosis (see Table 1). The molecular basis for LAD was first suggested by Crowley and colleagues, who found that neutrophils from a patient with this clinical syndrome lacked a high-molecular-weight membrane glycoprotein (see Dinauer, Newburger, and Borregaard). The patient’s neutrophils could not be made to adhere to plastic surfaces or to respond to serum-opsonized particles in terms of ingestion and respiratory burst activity.
Table1. Summary of Leukocyte Adhesion Deficiency Type 1
The molecular basis of LAD I is now known to result from mutations in the gene for the common CD18 β2 subunit for these three leukocyte glycoproteins, now termed β2 integrins, that belong to the integrin superfamily of adhesion molecules. Integrins are noncovalently linked heterodimeric glycoproteins consisting of an α and a β subunit. Within each of the eight known integrin subfamilies, the β subunit is identical (and defines the subfamily), but the α subunit varies and confers the functional specificity on the integrin. The molecular defect in LAD involves all members of the β2 integrin subfamily: αL β2 (CD11a/CD18), αm β2 (CD11b/ CD18), and α× β2 (CD11c/CD18). CD11a/CD18 is often referred to as LFA-1, and CD11b/CD18 is also called Mac-1, Mo1, or CR3. LAD I patients have an absent, diminished, or structurally abnormal β2 subunit (CD18; see later), and as a result, the three types of α chains in the β2 integrin subfamily cannot assemble into normal α–β heterodimers. Thus all three β2 integrins are moderately to severely deficient on all leukocytes in LAD.
The β2 integrins serve as receptors for the opsonic complement fragment C3bi, the intercellular adhesion molecules 1 and 2 (ICAM-1 and ICAM-2) that are expressed on endothelial cells and leukocytes, and fibrinogen. The diminished or absent expression of β2 integrins in LAD I leukocytes results in the failure of phagocytes to emigrate from the bloodstream to sites of infection. The early interactions with the endothelium, termed rolling, are normal in LAD I because these are mediated by a different family of adhesion molecules known as selectins. However, β2 integrins are responsible for the subsequent tight binding of neutrophils and monocytes to ICAMs on cytokine activated endothelium, and this step is therefore severely defective in LAD I. Transendothelial migration is also impaired. A second major functional defect in LAD is the failure of phagocytes to bind C3bi opsonized microbes. Because CD11b/CD18 is the predominant phagocyte receptor for this complement fragment, C3bi-mediated ingestion, degranulation, and respiratory burst activity are severely affected in LAD. Finally, β2 integrin–dependent signals play a key role in activating neutrophils for enhanced migration, phagocytosis of antibody-opsonized microbes, and degranulation.
Despite in vitro defects in lymphocyte responses dependent on LFA-1 (CD11a/CD18), patients with LAD I rarely have clinical manifestations related to impaired lymphocyte function. It is believed that the role CD11a/CD18 plays in lymphoid cell function can be compensated by other adhesion proteins (CD2, CD4, CD8, and so on).
Molecular Genetics of Leukocyte Adhesion Deficiency Type I
The fact that LAD I involves a deficiency of all leukocyte β2 integrins focused attention on the common β2 chain (CD18), and mutations in the corresponding gene, ITGB2, have been identified in all LAD I patients who have been analyzed at the molecular level to date. Although expression of the leukocyte integrin α subunits is normal in LAD I, these are not transported to the cell surface because the β2 chain is absent or contains mutations that disrupt its structure or its interaction with the α subunit. Mutations in the α subunits have not been found thus far in patients with LAD I. The CD18 glycoprotein has a large extracellular domain at the N terminus, a single transmembrane domain, and a 46-residue cytoplasmic tail. CD18 mutations in LAD I are heterogeneous in nature and family specific and can lead to either undetectable or low (9% to 20% of normal) levels of α–β dimer expression that correlates with the clinical severity of the disease. More than 50 different mutations have now been characterized in more than 100 families. These include missense mutations, messenger RNA (mRNA) splicing defects, small deletions, and a premature termination signal. Many patients are compound heterozygotes and have two different mutant alleles for CD18. About half of patients with LAD I in whom the genetic defect has been identified have point mutations in a stretch of 250 amino acids in the extracellular domain of CD18. This region is highly conserved among all β subunits and appears to be important for interaction with the α subunit.
Clinical Features
The key features of LAD I are summarized in Table 1. The clinical presentation of LAD is heterogeneous and is related to the severity of the deficiency of the β2 integrins. The severe clinical phenotype is associated with less than 0.3% of the normal amount of these glycoproteins on the leukocyte surface; the moderate phenotype has 2.5% to 6% of normal levels. In both the severe and moderate forms of the disease, persistent granulocytosis (neutrophil count of 12,000 to 100,000/mm3) is a constant finding, as are recurrent cutaneous abscesses and aggressive periodontitis and gingivitis. Recent studies suggest that impaired migration and clearance of neutrophils in periodontal tissues results in a dysregulated interleukin (IL)-23 to IL-17 inflammatory axis, which drives the severe periodontal disease typical of LAD I rather than impaired control of gum bacteria. Additional clinical features seen more often in the severe clinical phenotype include delayed umbilical cord separation, omphalitis, perirectal cellulitis, severe ulcerative stomatitis, and bacterial sepsis. A striking finding in LAD I is that abscesses and other sites of infections are devoid of pus despite the marked neutrophilia because neutrophils are unable to emigrate to tissues. S. aureus and gram-negative enteric bacteria cause the majority of infections in LAD I. Fungal infections can also occur, particularly from C. albicans and Aspergillus spp.
Note that infants with delayed separation of the umbilical cord who are healthy and have normal blood counts are very unlikely to have LAD I. Although the mean age of cord separation ranges from 7 to 15 days, 10% of healthy infants can have cord separation at 3 weeks of age or later.
Diagnosis
The diagnosis of LAD I is made by flow cytometric measurement of surface CD11b (Mac1; or the shared CD18 subunit) in unstimulated and stimulated neutrophils using commercially available monoclonal antibodies directed against CD11b or CD18 (Fig. 1). Neutrophils contain an intracellular pool of CD11b/CD18 in their secondary (specific) and tertiary granules, which can be mobilized to the cell surface during stimulation. Therefore the deficiency of CD11b can be more dramatically demonstrated by using stimulated neutrophils. Carriers of LAD I can be identified by this method because they have been found to express approximately 50% of normal levels of CD11b on the surface of their stimulated neutrophils (see Fig. 1).
Fig1. EVALUATION OF ADHESION MOLECULE EXPRESSION FOR DIAGNOSIS OF LEUKOCYTE ADHESION DEFICIENCY. Fluorescence of C3b-specific antibody labeled neutrophils (solid line) increases compared with a nonspecific control (dashed line) after 30-minute exposure to 10-nM N-formylmethione-leucyl-phenylalanine (fMLP) in healthy donors, patients with severe and moderate forms of leukocyte adhesion deficiency (LAD) I, and a heterozygous LAD I carrier. However, the increase in CD11b fluorescence (solid line) is markedly diminished in LAD I patients. The respective percent of normal stimulated mean channel number is shown in the right column. Note that these results are expressed as percent of normal fluorescence intensity, not as percent of positive cells, as is the case in most flow cytometry assays.
Prognosis and Treatment
Treatment of LAD I depends on the clinical severity of the disorder. In patients with the moderate clinical phenotype, cutaneous and oral infections can be managed as they occur. The use of prophylactic anti biotics such as TMP-SMX appears to be beneficial, as does aggressive prophylactic treatment of periodontal disease, which could also potentially benefit from agents targeting the IL-23 axis. It is important to note that even patients with the moderate phenotype can die of overwhelming infection. In patients with severe LAD I, aggressive management is indicated because of the high incidence of death before the age of 2 years, and HSCT is recommended. LAD I should also be amenable to gene-replacement therapy in the future.
