Antibody and complement-dependent viral neutralization

Evaluation of the Galalpha1-3Gal epitope as a host modification factor eliciting natural humoral immunity to enveloped viruses

Human sera contain high levels of natural antibody (Ab) to Galalpha1-3Gal, a terminal glycosidic structure expressed on the surface of cells of mammals other than Old World primates. Incorporation of this determinant onto retroviral membranes by passage of viruses in cells encoding alpha-1-3-galactosyltransferase (GT) renders retroviruses sensitive to lysis by natural Ab and complement in normal human serum (NHS). Plasma membrane-budding viruses representing four additional virus groups were examined for their sensitivities to serum inactivation after passage through human cell lines that lack a functional GT or human cells expressing recombinant porcine GT. The inactivation of lymphocytic choriomeningitis virus (LCMV) by NHS directly correlated with host modification of the virus via expression of Galalpha1-3Gal and was blocked by incorporation of soluble Galalpha1-3Gal disaccharide into the inactivation assay. GT-deficient mice immunized to make high levels of Ab to Galalpha1-3Gal (anti-Gal Ab) were tested for resistance to LCMV passaged in GT-expressing cells. Resistance was not observed, but in vitro analyses of the mouse immune sera revealed that the antiviral activity of the sera was insufficient to eliminate LCMV infectivity on its natural targets of infection, macrophages, which express receptors for Ab and complement. Newcastle disease virus and vesicular stomatitis virus (VSV) were inactivated by NHS regardless of cell passage history, whereas Sindbis virus (SV) passaged in human cells resisted inactivation. Both VSV and SV passaged in Galalpha1-3Gal-expressing human cells incorporated this sugar moiety onto their major envelope glycoproteins. SV passaged in mouse cells expressing Galalpha1-3Gal was moderately sensitive to inactivation by NHS. These results indicate that enveloped viruses expressing Galalpha1-3Gal differ in their sensitivities to NHS and that a potent complement source, such as that in NHS, is required for efficient inactivation of sensitive viruses in vitro and in vivo.

Transfer of host T-cell membrane HLA-DR and CD25 to target cells by human retroviruses

Many enveloped viruses incorporate host membrane proteins, some of which remain functionally active and significantly affect viral phenotype. We investigated whether human retroviruses can transfer host membrane proteins to target cells. Following incubation with HTLV-I, HLA-DR and CD25 were detected on up to 70% of HPB-ALL cells. Similarly, HLA-DR and CD25 were also detected on cells following incubation with HIV-1. Cyclohexamide or azidothymidine (AZT) had no effect on detection, indicating that binding of virus or infection did not induce expression of these proteins. Detection of host proteins on target cells depended on binding as well as fusion of virus to the cell membrane, indicating that these proteins were inserted into target cell membranes. Virions also transferred host proteins to peripheral blood mononuclear cells (PBMCs). This aberrant transfer of T-cell activation proteins by HIV or HTLV may alter the state of activation or proliferation of target cells and contribute to the immunodeficiencies associated with infection by these viruses.

Genetic predisposition to acute lymphocytic leukemia in American blacks. A Pediatric Oncology Group study

Recent reports have shown an association between genes lying within the major histocompatibility complex (MHC), particularly HLA and factor B (Bf), and acute lymphocytic leukemia (ALL) in white children. The frequencies of Bf and complement component C4 phenotypes in 90 black American children with ALL were examined to determine if a genetic association existed. The Bf and C4 results for the black children with ALL were compared with frequencies in healthy black Americans from the same geographic region. The BfF allele was carried by 95.6% of the black ALL patients compared with 86.1% of the controls (P = 0.017; relative risk = 3.5). In contrast, only 2.2% of the patients with ALL were homozygous for BfS compared with 9.8% of the controls (P = 0.043; relative risk = 0.2). These findings are similar to those observed in white American children. The C4A6 phenotype was found in 11.9% of the black children with ALL compared with 0.6% of the controls (P = 0.0026; relative risk = 22.7). These findings represent the first reported association of a particular allele whose gene lies within the MHC with ALL in black American children. The results suggest that the occurrence of ALL in black American children may be partially due to a genetic influence.

The role of antibody and complement in the control of viral infections

Host defense against viral infection is extremely complex and includes both humoral and cellular immune mechanisms. This contribution examines the mechanisms by which antibody (Ab) and the complement (C) system, major constituents of the humoral immune system, inactivate viruses and block viral maturation in virus-infected cells in vitro. Ab and C may neutralize viruses by envelopment in a coating of protein, by aggregation by lysis, or by facilitating interactions with various effector cells. Ab and C molecules deposited on the surfaces of viruses may physically interfere with the ability of the virus to infect a potentially susceptible cell. This appears to be the most common mechanism by which Ab and C neutralize viruses. In rare instances, Ab and/or C may aggregate viruses; aggregation reduces the net number of infectious particles and thus is manifest as neutralization. C may lyse enveloped viruses, resulting in irreversible viral inactivation. However, this does not appear to be a major mechanism of viral neutralization. Finally, the Fc portions of bound Ab molecules as well as bound C molecules may interact with effector cells with specific receptors for these factors and thereby facilitate viral destruction. In regard to virus-infected cells, the deposition of Ab or C on the cell surface may prevent the maturation or release of viral particles and alter normal cellular functions. Ab and C may also lyse virus-infected cells, abruptly stopping further viral maturation. Such lytic events require only the F(ab')2 portion of the Ab molecule and proceed via activation of the alternative C pathway. Effector cells may also interact with Ab and/or C molecules deposited on virus-infected cells, leading to cytotoxic reactions and/or ingestion depending on the type of effector cell involved. The activated C system has the ability to produce an acute inflammatory response leading to alterations in vessel permeability, edema, changes in smooth-muscle contractility, and the influx of leukocytes. Such inflammatory responses occurring in tissues, including the skin, as a result of C activation not only retard the spread of the infection and facilitate the destruction of the infectious agent, but also in all likelihood damage normal tissues in the vicinity. In addition, C activation in tissues also has the ability to stimulate arachidonic acid metabolism and induce the release of histamine and other mediators as well as pyrogens from appropriate cell types. A number of the systemic symptoms characteristic of viral infections, such as headaches, myalgias, and fever, likely result from such processes.

Partial C4 deficiency in subacute sclerosing panencephalitis

In an immunogenetic study, 23 subacute sclerosing panencephalitis (SSPE) patients and their families were studied for the HLA region markers HLA-A, B, C, DR, BF, C2, C4A, C4B, GLO I, and PGM3. In addition, C3, C4, and factor B serum levels were determined. A highly significant association of C4A QO with SSPE was found. Furthermore, two rare haplotypes, C4A QOB QO, two C4ACh+ allotypes, and four Ch partial inhibitors were detected, which possibly impair the function of the C4 molecules. HLA-DR5 was increased. In addition, a number of rare HLA-A, C, B, DR haplotypes were observed. It is postulated that rare C4 molecular deficiency might be a predisposing factor in the pathogenesis of SSPE.

Complement, viruses, and virus-infected cells

The attachment of specific antibody to viral glycoproteins and other structures on the surface of a virus or virus-infected cell has a number of potential consequences to the virus or virus-infected cell. Antibody is multivalent and thus able to redistribute or patch surface viral proteins or virus-encoded structures within the lipid bilayer of the viral envelope or the cell membrane. In certain instances, antibody may agglutinate viruses or virus-infected cells. The physical presence of antibody molecules on the virus surface may interfere with the ability of the virus to infect potentially susceptible cells. Antibody on the surface of virus-infected cells may prevent the maturation and release of virus particles; antibody also can alter certain normal cell functions. The Fc portions of antibody molecules bound to virus-infected cells facilitate interactions with effector cells bearing Fc receptors. In the case of lymphocytes and perhaps phagocytic cells, this interaction may lead to antibody-dependent cellular cytotoxicity (ADCC) [51, 58]. The exposed Fc regions may also facilitate attempts at ingestion by monocytes, macrophages, and polymorphonuclear leukocytes.

On the significance of C2, C4, and factor B polymorphisms in disease

In this review article, recent evidence is presented that some diseases like insulin-dependent diabetes mellitus, multiple sclerosis, and idiopathic membranous nephropathy, which are primarily associated with HLA-D,DR, are also related to the rare C2, C4, and Factor B alleles. Circumstantial evidence is available that at least some of these rare variants may be functionally deficient. Based on the concept of functionally interacting gene clusters, mutant complement genes may lead to impaired effector mechanisms in virus neutralization or lysis of virus-infected cells. Other mechanisms such as alteration of vascular permeability may be involved in the development of proliferative retinopathy and familial hypertension. In lepromatous lepra, an impaired cell-mediated lysis of M. leprae may be related to the hemolytically inactive C4F1 allelic product.