Initial biocompatibility studies of a novel degradable polymeric bone substitute that hardens in situ

Clinical management of osseous defects often requires bone grafts. The standard for treatment is autogenous bone harvested from sites such as either the iliac crest or the outer table of the calvaria. In addition to the problem of donor site morbidity and the limited supply of graft material, there is the additional operating time associated with harvesting procedures. A synthetic, bone graft substitute that can match the clinical performance of autogenous bone could alleviate these deficiencies. Therefore, a polymeric bone substitute was developed that consists of a four-armed star polymer of poly(dioxanone-co-glycolide) endcapped at each termini with a biocompatible lysine-based diisocyanate crosslinker. The polymer can be mixed with inorganic fillers such as either hydroxyapatite or tricalcium phosphate to form either injectable or moldable putty. The addition of a catalyst (for example, diethylaminoethanol and water) to the polymer produces a crosslinking reaction causing the combination to harden. This reaction is nontoxic, normo-thermic and can be performed in situ. During the course of the polymerization, carbon dioxide is liberated, producing an interconnected porous network within the implant, suitable for bone ingrowth. This paper will describe a preliminary biocompatibility assay of the bone substitute.

[Self-management by psychiatrically experienced persons]

As we all-including psychiatrists-are only able to achieve true understanding of that which we have already experienced, the complete understanding of a psychosis can only be developed by the sufferers themselves. Starting in 1989 we began to organize "psychosis workshops" at the university of Hamburg in order to exchange experiences. In the meantime there are an additional of 70 "psychosis workshops" offered throughout Germany. Conversely, the "Federal Association" founded in 1992 of the same name was the result of the "Psychosis Workshop" initiated by us in 1989.

Investigation of the factor VIII intron 22 repeated region (int22h) and the associated inversion junctions

A region of intron 22 of the factor VIII gene, which contains factor VIII-associated gene A (F8A), is repeated twice more nearer the Xq telomere. It has been proposed that intrachromosomal homologous recombination occurs between the intron 22 repeat and either of the two extragenic copies, resulting in the recurrent inversions that cause almost half of all cases of severe haemophilia A. We have precisely defined the repeated region as 9.5 kb of DNA which we have termed int22h (intron 22 homologous region). The junctions of the inversions examined were shown to represent precise exchanges between the int22h repeats, thus providing conclusive evidence for homologous recombination. The three copies of int22h were compared along 8 kb of their length, using chemical mismatch analysis, and found to be 99.9% similar. The presence of such long, almost identical inverted repeats near the Xq telomere could account for the high frequency at which the inversions occur.

Comparative sequence analysis of the human and pufferfish Huntington's disease genes

The Huntington's disease (HD) gene encodes a novel protein with as yet no known function. In order to identify the functionally important domains of this protein, we have cloned and sequenced the homologue of the HD gene in the pufferfish, Fugu rubripes. The Fugu HD gene spans only 23 kb of genomic DNA, compared to the 170 kb human gene, and yet all 67 exons are conserved. The first coding exon, the site of the disease-causing triplet repeat, is highly conserved. However, the glutamine repeat in Fugu consists of just four residues. We also show that gene order may be conserved over longer stretches of the two genomes. Our work describes a detailed example of sequence comparison between human and Fugu, and illustrates the power of the pufferfish genome as a model system in the analysis of human genes.

Identification of a human OX-40 ligand, a costimulator of CD4+ T cells with homology to tumor necrosis factor

The human OX-40 cell surface antigen is a CD4+ T cell activation marker that acts as a costimulatory receptor and is a member of the nerve growth factor receptor/tumor necrosis factor (TNF) receptor family. Using a soluble form of the receptor, the extracellular region fused with human immunoglobulin Fc, we expression cloned the human OX-40 ligand cDNA from a library derived from an activated B lymphoblastoid cell line MSAB. The encoded protein is identified as gp34, a type II transmembrane antigen previously known to be expressed only by human T cell lymphotropic virus 1-infected cells. We describe gp34 as a new member of the TNF family, and find that the recombinant ligand expressed in COS cells costimulates phorbol myristate acetate, phytohemagglutinin, and anti-CD3-induced CD4+ T cell proliferation.

Quantitative light microscopy. A powerful tool to assess bone

Quantitative light microscopy is an important tool for assessing objectively the status of histologic events. It is, therefore, extremely useful for studying the dynamics of bone. This article provides a brief description of three quantitative techniques that are applicable to histomorphometry.

Selective expression of integrin alpha 4 beta 7 on a subset of human CD4+ memory T cells with Hallmarks of gut-trophism

Human memory CD4+ T lymphocytes are heterogenous in expression of integrins; one subset has the unexpected phenotype beta 1 low alpha 4 high. We demonstrate that this subset is unique among CD4+ cells in expression of high levels of alpha 4 beta 7, detected by a distinctive mAb Act-1. alpha 4 beta 7 is involved in binding to both fibronectin and vascular cell adhesion molecule-1; Act-1 blocks cell binding to the former and augments binding to the latter. Act-1 expression marks a subset of memory cells that, unlike the predominant circulating memory cell, has up-regulated beta 7 rather than beta 1. Their phenotype is distinct from that described for skin-homing T cells and is fully consistent with that described for gut-homing T cells. Differential adhesion capacity of this subset is verified by selective binding to FN and vascular cell adhesion molecule-1 in a beta 1-independent fashion. Thus, alpha 4 beta 7 detected on this subset of circulating normal T cells fits the expectations for a gut-homing receptor.

Selective expression of integrin alpha 4 beta 7 on a subset of human CD4+ memory T cells with Hallmarks of gut-trophism

Human memory CD4+ T lymphocytes are heterogenous in expression of integrins; one subset has the unexpected phenotype beta 1 low alpha 4 high. We demonstrate that this subset is unique among CD4+ cells in expression of high levels of alpha 4 beta 7, detected by a distinctive mAb Act-1. alpha 4 beta 7 is involved in binding to both fibronectin and vascular cell adhesion molecule-1; Act-1 blocks cell binding to the former and augments binding to the latter. Act-1 expression marks a subset of memory cells that, unlike the predominant circulating memory cell, has up-regulated beta 7 rather than beta 1. Their phenotype is distinct from that described for skin-homing T cells and is fully consistent with that described for gut-homing T cells. Differential adhesion capacity of this subset is verified by selective binding to FN and vascular cell adhesion molecule-1 in a beta 1-independent fashion. Thus, alpha 4 beta 7 detected on this subset of circulating normal T cells fits the expectations for a gut-homing receptor.

Cloning and sequencing of the Escherichia coli panB gene, which encodes ketopantoate hydroxymethyltransferase, and overexpression of the enzyme

The panB gene from Escherichia coli, encoding the first enzyme of the pantothenate biosynthesis pathway, ketopantoate hydroxymethyltransferase (KPHMT), has been isolated by functional complementation of a panB mutant strain with an E. coli genomic library. The gene is 792 bp long, encoding a protein of 264 amino acids with a predicted M(r) of 28,179. The identity of the gene product as ketopantoate hydroxymethyltransferase was confirmed by purification of the enzyme protein, which was overexpressed approximately 50-fold in the mutant harboring the gene on a high-copy-number plasmid. The N-terminal amino acid sequence of the purified protein was found to be identical to that predicted from the gene sequence, as was its mass, determined by electrospray mass spectrometry. Upstream of the panB gene is an incomplete open reading frame encoding a protein of 220 amino acids, which shares sequence similarity to fimbrial precursor proteins from other bacteria. Northern (RNA) analysis showed that the panB gene is likely to be cotranscribed with at least one other gene but that this is not the putative fimbrial protein, since no transcripts for this gene could be detected.

Control of lymphocyte recirculation in man. I. Differential regulation of the peripheral lymph node homing receptor L-selectin on T cells during the virgin to memory cell transition

Conventional virgin lymphocytes of a given class show relatively homogeneous recirculation through secondary lymphoid tissues, whereas memory/effector populations are composed of distinct subsets with differential, often tissue-selective migratory capability. In keeping with these observations, CD45RAhigh/ROlow "virgin" T cells in human peripheral blood uniformly express the peripheral lymph node homing receptor (HR) L-selectin, whereas among the CD45RAlow/ROhigh "memory/effector" subset, the expression of this HR is bimodal. To investigate the mechanisms responsible for the generation of memory/effector T cell subsets with differential homing potential, we developed a multiparameter flow cytometric technique that defines a common pathway of post-thymic T cell differentiation in secondary lymphoid tissues. Our analyses indicate that the virgin to memory T cell transition is characterized by a stepwise, unidirectional progression through distinct CD45RA+/RO+ intermediates that allow the in vivo discrimination of early, middle, and late transitional T cells. In normal peripheral blood, few T cells with a transitional phenotype are identified, but in secondary lymphoid tissues, 2 to 10% of T cells have this phenotype, including those CD45RA+ T cells that 1) are morphologically blasts, 2) are in S or G2/M phase of the cell cycle, or 3) express activation Ag. General adhesion molecules (LFA-1, LFA-3, ICAM-1) are uniformly up-regulated concordant with changes in T cell expression of CD45RA/RO in all tissues examined. Early in the transition, L-selectin is also uniformly up-regulated, but in subsequent stages, T cell expression of this HR is preferentially down-regulated in mucosal lymphoid tissues, and retained in peripheral lymph node. Differential regulation of L-selectin can also be demonstrated in vitro by the activation of virgin T cells in the presence of specific cytokines--IL-2 induces L-selectin down-regulation, whereas IL-6 and particularly TGF-beta 1 promote L-selectin up-regulation. Taken together, these findings support the hypothesis that local microenvironments within particular secondary lymphoid tissues influence HR expression at the time of CD45RA/RO conversion, and thereby contribute to the formation of CD45RAlow/ROhigh memory/effector T cell populations with tissue-selective homing behavior.

Control of lymphocyte recirculation in man. I. Differential regulation of the peripheral lymph node homing receptor L-selectin on T cells during the virgin to memory cell transition

Conventional virgin lymphocytes of a given class show relatively homogeneous recirculation through secondary lymphoid tissues, whereas memory/effector populations are composed of distinct subsets with differential, often tissue-selective migratory capability. In keeping with these observations, CD45RAhigh/ROlow "virgin" T cells in human peripheral blood uniformly express the peripheral lymph node homing receptor (HR) L-selectin, whereas among the CD45RAlow/ROhigh "memory/effector" subset, the expression of this HR is bimodal. To investigate the mechanisms responsible for the generation of memory/effector T cell subsets with differential homing potential, we developed a multiparameter flow cytometric technique that defines a common pathway of post-thymic T cell differentiation in secondary lymphoid tissues. Our analyses indicate that the virgin to memory T cell transition is characterized by a stepwise, unidirectional progression through distinct CD45RA+/RO+ intermediates that allow the in vivo discrimination of early, middle, and late transitional T cells. In normal peripheral blood, few T cells with a transitional phenotype are identified, but in secondary lymphoid tissues, 2 to 10% of T cells have this phenotype, including those CD45RA+ T cells that 1) are morphologically blasts, 2) are in S or G2/M phase of the cell cycle, or 3) express activation Ag. General adhesion molecules (LFA-1, LFA-3, ICAM-1) are uniformly up-regulated concordant with changes in T cell expression of CD45RA/RO in all tissues examined. Early in the transition, L-selectin is also uniformly up-regulated, but in subsequent stages, T cell expression of this HR is preferentially down-regulated in mucosal lymphoid tissues, and retained in peripheral lymph node. Differential regulation of L-selectin can also be demonstrated in vitro by the activation of virgin T cells in the presence of specific cytokines--IL-2 induces L-selectin down-regulation, whereas IL-6 and particularly TGF-beta 1 promote L-selectin up-regulation. Taken together, these findings support the hypothesis that local microenvironments within particular secondary lymphoid tissues influence HR expression at the time of CD45RA/RO conversion, and thereby contribute to the formation of CD45RAlow/ROhigh memory/effector T cell populations with tissue-selective homing behavior.

Characteristics of murine monoclonal anti-CD4. Epitope recognition, idiotype expression, and variable region gene sequence

We have characterized a series of mouse monoclonal anti-CD4 and describe both their CD4 epitope recognition and Id expression. We also determined the V region gene sequences of these antibodies in an attempt to correlate epitope recognition and Id expression with V region sequence. All of these preparations recognize epitopes that cluster around the HIV gp120 binding site on the human CD4 molecule. However, we observed differences in epitope recognition among the anti-CD4 preparations, based on either competitive inhibition assays or functional assays, such as syncytium inhibition. Analysis of Id specificities using a polyclonal anti-Id generated against anti-Leu 3a indicated that five of the seven monoclonal anti-CD4 expressed a shared Id. Based on V region gene sequences, the V region kappa-chain (V[kappa]) from each of the seven antibodies was encoded by the V[kappa]21 gene family and expressed the J[kappa]4 gene segment. Those preparations that expressed the shared Id with anti-Leu 3a have virtually identical V[kappa] sequences, with a high degree of homology in the CDR. The VH region gene sequences of six of the seven antibodies also shared overall homology and appeared to be encoded by the J558 VH gene family. The seventh anti-CD4 VH region is encoded for by the VHGAM gene family. The majority of these antibodies used JH3 gene segment, although the JH2 and JH4 gene segments were also represented. In addition, several of these antibodies share a common sequence organization within their V-D-J joining regions that appears to involve N and P sequences to generate unique D segments. Together, these data suggest that differences in epitope recognition among the monoclonal anti-CD4 may reflect sequence variability primarily within the CDR3 region of both V[kappa] and VH. The basis for the detection of a shared Id most likely reflects the high degree of homology within the V[kappa] region sequences. In addition, these data, which are based on a limited analysis, suggest the possible restricted use of V region germ-line gene families in the secondary antibody response of BALB/c mice to specific epitopes on the human CD4 molecule.

Characteristics of murine monoclonal anti-CD4. Epitope recognition, idiotype expression, and variable region gene sequence

We have characterized a series of mouse monoclonal anti-CD4 and describe both their CD4 epitope recognition and Id expression. We also determined the V region gene sequences of these antibodies in an attempt to correlate epitope recognition and Id expression with V region sequence. All of these preparations recognize epitopes that cluster around the HIV gp120 binding site on the human CD4 molecule. However, we observed differences in epitope recognition among the anti-CD4 preparations, based on either competitive inhibition assays or functional assays, such as syncytium inhibition. Analysis of Id specificities using a polyclonal anti-Id generated against anti-Leu 3a indicated that five of the seven monoclonal anti-CD4 expressed a shared Id. Based on V region gene sequences, the V region kappa-chain (V[kappa]) from each of the seven antibodies was encoded by the V[kappa]21 gene family and expressed the J[kappa]4 gene segment. Those preparations that expressed the shared Id with anti-Leu 3a have virtually identical V[kappa] sequences, with a high degree of homology in the CDR. The VH region gene sequences of six of the seven antibodies also shared overall homology and appeared to be encoded by the J558 VH gene family. The seventh anti-CD4 VH region is encoded for by the VHGAM gene family. The majority of these antibodies used JH3 gene segment, although the JH2 and JH4 gene segments were also represented. In addition, several of these antibodies share a common sequence organization within their V-D-J joining regions that appears to involve N and P sequences to generate unique D segments. Together, these data suggest that differences in epitope recognition among the monoclonal anti-CD4 may reflect sequence variability primarily within the CDR3 region of both V[kappa] and VH. The basis for the detection of a shared Id most likely reflects the high degree of homology within the V[kappa] region sequences. In addition, these data, which are based on a limited analysis, suggest the possible restricted use of V region germ-line gene families in the secondary antibody response of BALB/c mice to specific epitopes on the human CD4 molecule.

Preoperative radiological evaluation of the liver by computerized tomographic portography in patients with hepatic tumors

The main objective of preoperative imaging studies is to define as accurately as possible the number, size, location, and relationship of tumor masses in the liver to pertinent portal and hepatic venous vasculature. Computerized tomographic portography images hepatic veins and segmental portal vein branches and identifies the anatomical location of tumor nodules with excellent sensitivity and a low false-positive rate. The intraoperative correlation of computerized tomographic portography on 30 patients in the last 20 months at this institution shows a sensitivity of 88 per cent with a low rate of false-positivity. The ability to detect metastatic lesions in the liver by computerized tomographic portography diminishes when the lesions are noted to be less than 1 cm. The authors conclude that the preoperative interpretation of the computerized tomographic portogram provides valuable information not previously available to the surgeon operating on the liver.

Involvement of CD28 in MHC-unrestricted cytotoxicity mediated by a human natural killer leukemia cell line

NK cells and certain CTL can recognize and lyse targets without restriction by the MHC. NK cells do not express CD3/TCR complexes and the membrane receptors participating in MHC-unrestricted cytotoxicity are largely unknown. We demonstrate that YT2C2, a human NK leukemia cell line, expresses the CD28 differentiation Ag and can spontaneously lyse both murine and human cell lines expressing B7, a B cell- activation Ag that is a ligand for CD28. The participation of CD28/B7 interactions in MHC-unrestricted cytotoxicity mediated by YT2C2 cells was demonstrated by correlation of target sensitivity with levels of B7 expression, inhibition of cytotoxicity by anti-CD28 or anti-B7 mAb, and by making both murine and human cell lines susceptible to YT2C2-mediated lysis by genetic transfection with expression vectors containing B7 cDNA. However, CD28/B7 interactions alone were insufficient to initiate cytotoxicity. mAb inhibition experiments and selection of CD54- (intercellular adhesion molecule-1) deficient B cell targets indicated that CD11a/18 (lymphocyte function-associated Ag-1) also cooperated in CD28/B7-dependent cytotoxicity. The requirement for both CD28/B7 and lymphocyte function-associated Ag-1/intercellular adhesion molecule-1 interactions in YT2C2-mediated MHC-unrestricted cytotoxicity was confirmed by demonstrating that efficient lysis of murine L cells required cotransfection with both B7 and intercellular adhesion molecule-1. These findings support the concept that MHC-unrestricted cytotoxicity may not be due to a unique receptor, but may result from interactions between an appropriate array of "adhesion" molecules with their ligands.

Involvement of CD28 in MHC-unrestricted cytotoxicity mediated by a human natural killer leukemia cell line

NK cells and certain CTL can recognize and lyse targets without restriction by the MHC. NK cells do not express CD3/TCR complexes and the membrane receptors participating in MHC-unrestricted cytotoxicity are largely unknown. We demonstrate that YT2C2, a human NK leukemia cell line, expresses the CD28 differentiation Ag and can spontaneously lyse both murine and human cell lines expressing B7, a B cell- activation Ag that is a ligand for CD28. The participation of CD28/B7 interactions in MHC-unrestricted cytotoxicity mediated by YT2C2 cells was demonstrated by correlation of target sensitivity with levels of B7 expression, inhibition of cytotoxicity by anti-CD28 or anti-B7 mAb, and by making both murine and human cell lines susceptible to YT2C2-mediated lysis by genetic transfection with expression vectors containing B7 cDNA. However, CD28/B7 interactions alone were insufficient to initiate cytotoxicity. mAb inhibition experiments and selection of CD54- (intercellular adhesion molecule-1) deficient B cell targets indicated that CD11a/18 (lymphocyte function-associated Ag-1) also cooperated in CD28/B7-dependent cytotoxicity. The requirement for both CD28/B7 and lymphocyte function-associated Ag-1/intercellular adhesion molecule-1 interactions in YT2C2-mediated MHC-unrestricted cytotoxicity was confirmed by demonstrating that efficient lysis of murine L cells required cotransfection with both B7 and intercellular adhesion molecule-1. These findings support the concept that MHC-unrestricted cytotoxicity may not be due to a unique receptor, but may result from interactions between an appropriate array of "adhesion" molecules with their ligands.

CD28 interaction with B7 costimulates primary allogeneic proliferative responses and cytotoxicity mediated by small, resting T lymphocytes

Engagement of the CD3/T cell antigen receptor complex on small, resting T cells is insufficient to trigger cell-mediated cytotoxicity or to induce a proliferative response. In the present study, we have used genetic transfection to demonstrate that interaction of the B7-BB1 B cell activation antigen with the CD28 T cell differentiation antigen costimulates cell-mediated cytotoxicity and proliferation initiated by either anti-CD2 or anti-CD3 monoclonal antibody (mAb). Moreover, a B7- negative Burkitt's lymphoma cell line that fails to stimulate an allogeneic mixed lymphocyte response is rendered a potent stimulator after transfection with B7. The mixed leukocyte reaction proliferative response against the B7 transfectant is inhibited by either anti-CD28 or B7 mAb. We also demonstrate that freshly isolated small, resting human T cells can mediate anti-CD3 or anti-CD2 mAb-redirected cytotoxicity against a murine Fc receptor-bearing mastocytoma transfected with human B7. These preexisting cytotoxic T lymphocytes in peripheral blood are present in both the CD4 and CD8 subsets, but are preferentially within the CD45RO+ "memory" population. While small, resting T cells apparently require costimulation by CD28/B7 interactions, this requirement is lost after T cell activation. Anti- CD3 initiates a cytotoxic response mediated by in vitro cultured T cell clones in the absence of B7 ligand. The existence of functional cytolytic T cells in the small, resting T cell population may be advantageous in facilitating rapid responses to immune challenge.

Structural characterization of a cross-reactive idiotype shared by monoclonal antibodies specific for the human CD4 molecule

A panel of mouse monoclonal anti-CD4 antibodies was characterized in terms of idiotypic expression by using specific anti-idiotypic antibody (anti-Id) reagents generated in rabbits immunized with anti-Leu3a, a monoclonal anti-CD4 which inhibits the human immunodeficiency virus (HIV) gp120 binding to CD4. Direct binding and competitive inhibition assays demonstrate that the majority of monoclonal anti-CD4 antibodies able to recognize CD4 epitopes overlapping the epitope recognized by anti-Leu3a expressed an antigen-combining site-related cross-reactive idiotype (IdX). Western blot analysis was used to demonstrate that this IdX is associated primarily with the light (L) chain of the monoclonal anti-CD4 antibodies. To further characterize the structural basis of the IdX, the nucleotide sequence of the variable region of the L kappa chain of anti-Leu3a was determined. Peptides corresponding to the first, second, and third complementarity determining regions (CDRs) of the L chain of anti-Leu3a were synthesized and used to immunize rabbits. All anti-peptide antisera recognized the immunizing peptide, the cognate anti-Leu3a molecule, and several other monoclonal anti-CD4 antibodies by direct binding assays. Western blot analysis utilizing the anti-CDR peptide reagents demonstrates that the reactivity to the monoclonal anti-CD4 antibodies was L chain-specific. The anti-Id generated by immunizing with the intact anti-Leu3a molecule failed to recognize the three L chain-derived CDR synthetic peptides, suggesting that the IdX requires the presence of the three-dimensional configuration of the L chain for its expression. The broad range of reactivity exhibited by the antipeptide antisera indicates that the majority of mouse monoclonal anti-CD4 antibodies characterized in this study utilize L chains encoded by a single germ line variable (V) region kappa (V kappa) chain gene or by V kappa genes that belong to the same gene family.

A highly selected panel of anti-CD4 antibodies fails to induce anti-idiotypic antisera mediating human immunodeficiency virus neutralization

Anti-CD4 antibodies directed to the N terminus of CD4 can inhibit human immunodeficiency virus (HIV) infection. Therefore, it has been proposed that some of these reagents may contain idiotypic determinants which conformationally model the binding site expressed on gp120. In this report, we have selected a panel of anti-CD4 monoclonal antibodies as idiotypic mimics of gp120 by employing cross-blocking techniques, and CD4 epitope mapping using site-directed mutagenesis. These studies suggest that only 4 out of the original panel of 12 would be expected to represent suitable candidates for modelling the gp120 binding site. Nevertheless, anti-idiotypic antisera raised against these antibodies failed to inhibit gp120 binding to CD4. This negative result may reflect the incomplete modelling of the virus binding site by anti-CD4, or the lack of internal image antibody in the anti-idiotypic preparations. Alternatively, the binding site on gp120 may not be accessible to antibody neutralization, excluding the possibility of an idiotypic vaccine to HIV based on anti-CD4 antibody as surrogate antigen.

A region in domain 1 of CD4 distinct from the primary gp120 binding site is involved in HIV infection and virus-mediated fusion

The high affinity binding site for human immunodeficiency virus (HIV) envelope glycoprotein gp120 resides within the amino-terminal domain (D1) of CD4. Mutational and antibody epitope analyses have implicated the region encompassing residues 40-60 in D1 as the primary binding site for gp120. Outside of this region, a single residue substitution at position 87 abrogates syncytium formation without affecting gp120 binding. We describe two groups of CD4 monoclonal antibodies (mAbs) which recognize distinct epitopes associated with these regions in D1. These mAbs distinguish between the gp120 binding event and virus infection and virus-induced cell fusion. One cluster of mAbs, which bind at or near the high affinity gp120 binding site, blocked gp120 binding to CD4 and, as expected, also blocked HIV infection of CD4+ cells and virus-induced syncytium formation. A second cluster of mAbs, which recognize the CDR-3 like loop, did not block gp120 binding as demonstrated by their ability to form ternary complexes with CD4 and gp120. Yet, these mAbs strongly inhibited HIV infection of CD4+ cells and HIV-envelope/CD4-mediated syncytium formation. The structure of D1 has recently been solved at atomic resolution and in its general features resembles IgVk regions as predicted from sequence homology and mAb epitopes. In the D1 structure, the regions recognized by these two groups of antibodies correspond to the C'C" (Ig CDR2) and FG (Ig CDR3) hairpin loops, respectively, which are solvent-exposed beta turns protruding in two different directions on a face of D1 distal to the D2 domain. This face is straddled by the longer BC (Ig CDR1) loop which bisects the plain formed by C'C'' and FG. This structure is consistent with C'C'' and FG forming two distinct epitope clusters within D1. We conclude that the initial interaction between gp120 and CD4 is not sufficient for HIV infection and syncytium formation and that CD4 plays a critical role in the subsequent virus-cell and cell-cell membrane fusion events. We propose that the initial binding of CD4 to gp120 induces conformational changes in gp120 leading to subsequent interactions of the FG loop with other regions in gp120 or with the fusogenic gp41 potion of the envelope gp160 glycoprotein.