The protection receptor for IgG catabolism is the beta2-microglobulin-containing neonatal intestinal transport receptor

More than 30 years ago, Brambell published the hypothesis bearing his name [Brambell, F. W. R., Hemmings, W. A. & Morris, 1. C. (1964) Nature (London) 203, 1352-1355] that remains as the cornerstone for thinking on IgG catabolism. To explain the long survival of IgG relative to other plasma proteins and its pattern of increased fractional catabolism with high concentrations of IgG, Brambell postulated specific IgG "protection receptors" (FcRp) that would bind IgG in pinocytic vacuoles and redirect its transport to the circulation; when the FcRp was saturated, the excess unbound IgG then would pass to unrestricted lysosomal catabolism. Brambell subsequently postulated the neonatal gut transport receptor (FcRn) and showed its similar saturable character. FcRn was recently cloned but FcRp has not been identified. Using a genetic knockout that disrupts the FcRn and intestinal IgG transport, we show that this lesion also disrupts the IgG protection receptor, supporting the identity of these two receptors. IgG catabolism was 10-fold faster and IgG levels were correspondingly lower in mutant than in wild-type mice, whereas IgA was the same between groups, demonstrating the specific effects on the IgG system. Disruption of the FcRp in the mutant mice was also shown to abrogate the classical pattern of decreased IgG survival with higher IgC concentration. Finally, studies in normal mice with monomeric antigen-antibody complexes showed differential catabolism in which antigen dissociates in the endosome and passes to the lysosome, whereas the associated antibody is returned to circulation; in mutant mice, differential catabolism was lost and the whole complex cleared at the same accelerated rate as albumin, showing the central role of the FcRp to the differential catabolism mechanism. Thus, the same receptor protein that mediates the function of the FcRn transiently in the neonate is shown to have its functionally dominant expression as the FcRp throughout life, resolving a longstanding mystery of the identity of the receptor for the protection of IgG. This result also identifies an important new member of the class of recycling surface receptors and enables the design of protein adaptations to exploit this mechanism to improve survivals of other therapeutic proteins in vivo.

Phosphoinositide 3-kinase and p72syk noncovalently associate with the low affinity Fc gamma receptor on human platelets through an immunoreceptor tyrosine-based activation motif. Reconstitution with synthetic phosphopeptides

Previously, we have demonstrated that the cytoplasmic tyrosine kinase p72syk is coupled to the platelet Fc receptor for IgG (Fc gamma RIIA) (Chacko, G. W., Duchemin, A. M., Coggeshall, K. M., Osborne, J. M., Brandt, J. T., and Anderson, C. L. (1994) J. Biol. Chem. 269, 32435-32440). Further analysis of the platelet activation by Fc gamma RIIA demonstrated that Fc gamma RIIA is also inducibly coupled to the serine/threonine and lipid kinase, phosphoinositide 3-kinase (PI 3-K). activation of platelets with anti-Fc gamma RIIA antibodies resulted in the noncovalent association of PI 3-K with Fc gamma RIIA as well as an increase in Fc gamma RIIA-associated PI 3-K activity. Binding of both p72syk and PI 3-K to Fc gamma RIIA was reconstituted with synthetic phosphopeptides corresponding to the sequence of the atypical immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic domain of Fc gamma RIIA. Our findings demonstrate that coupling of both p72syk and PI. 3-K activities to Fc gamma RIIA is regulated by tyrosine phosphorylation of the ITAM, and we speculate that p72syk might act as an adapter to recruit PI 3-K to activated Fc gamma RIIA.

A novel role for the Fc receptor gamma subunit: enhancement of Fc gamma R ligand affinity

The Fc receptors (FcR), which belong to the immunoglobulin (Ig) superfamily, bind to specific Ig isotypes with varying affinities triggering complex immune defense responses. Several of the FcR that lack signaling motifs in their cytoplasmic domains rely on associated subunits to transmit signals. Two classes of FcR that bind the Fc portion of IgG, Fc gamma RI, and Fc gamma RIIIa associate with a subunit shared among several FcR, the gamma chain, which is involved in receptor expression and signal transduction. In this report, we propose that a novel role for gamma chain is to enhance the affinity of Fc gamma R for ligand. Our findings demonstrate that Fc gamma RI requires gamma -chain association to attain high affinity binding for monomeric IgG, and suggest that the intermediate binding affinity of the Fc gamma RIIIa isoform results from its association with gamma chain. The affinity increase conferred by gamma chain appears to be mediated through the transmembrane domain of the Fc gamma R, with no requirement for the cytoplasmic domain of the receptor.

Pediatric injury hospitalization in Hispanic children and non-Hispanic white children in southern California

OBJECTIVE

To compare the incidence and causes of injury requiring hospitalization or resulting in death or both between Hispanic children and non-Hispanic white children.

DESIGN

Population-based surveillance of children younger than 15 years residing in eight Orange County cities and communities who were hospitalized or died of injuries sustained during 1991 and 1992.

SETTING

Eight hospitals and the coroner's office in central Orange County, California.

PARTICIPANTS

Study population was 213,906 children residing in the study area. Forty-nine percent were Hispanic, 37% were non-Hispanic white, 12% were Asian or Pacific Islander, and 3% were of other racial origin.

RESULTS

A total of 1361 severe injuries were identified (crude annual rate was 318 per 100,000 population). The crude incidence rate ratio comparing Hispanics and non-Hispanic whites was 1.82. After adjustment for census block group, Hispanic children had a 60% higher injury rate and incidence rate ratios of more than 2 for pedestrian injuries, asphyxia, aspirations, foreign-body ingestions, and poisonings.

CONCLUSIONS

Hispanic children had higher injury rates than non-Hispanic white children, even when controlling for census block group. These rate differences may be related to differences in exposure to various causes of injury, injury prevention practices, parenting practices, family size, and language. Injury rate differences by ethnicity that address specific injury hazards must be explored to guide prevention efforts. More culturally and linguistically appropriate interventions are needed to provide injury prevention programs to the Hispanic population.

The three genes of the human FCGR1 gene family encoding Fc gamma RI flank the centromere of chromosome 1 at 1p12 and 1q21

The high-affinity receptor for immunoglobulin G, Fc gamma RI (FCGR1), is encoded by a family of three genes within humans that share over 98% of DNA sequence homology. Efforts to define the location of the FCGR1 genes within chromosome 1 have been made to determine if they are tightly linked to the five other FCGR genes present at 1q23. Our results, obtained through both fluorescence in situ hybridization analysis of human cells and Southern analysis of cell lines containing 1p and 1q, show instead that the three genes flank the centromere of chromosome 1 at bands 1p12 and 1q21. FCGR1B was found at 1p12, whereas both FCGR1A and FCGR1C were localized to 1q21. This places the FCGR1 gene family within a large pericentric linkage group which is conserved between humans and mice. We hypothesize that the three FCGR1 genes were separated by a pericentric inversion known to have occurred on human chromosome 1, which relocated FCGR1A and FCGR1C to the long arm and left FCGR1B positioned on the short arm. We have also performed FCGR1 gene copy number experiments which indicate the existence of three FCGR1 genes within the human genome.

On the role of platelet Fc gamma RIIa phenotype in heparin-induced thrombocytopenia

Heparin induced thrombocytopenia (HIT) is characterized by the formation of antibodies that activate normal donor platelets in vitro in the presence of heparin. We asked whether the commonly observed donor-specific variation in the platelet aggregation response to HIT antibodies is influenced by the density of Fc gamma RII on platelets or by the Arg/His 131 allelic polymorphism of platelet Fc gamma RII. We found that platelets with His/His 131 Fc gamma RII phenotype were unresponsive to HIT antibody (0/9) whereas platelets with the Arg/Arg 131 phenotype responded well (7/9). His/His 131 platelets were largely unresponsive also to a murine IgG1 antiplatelet monoclonal antibody (UR1) known to activate platelets by Fc gamma RII clustering. We then determined the frequency distribution of Fc gamma RIIa Arg/His 131 phenotypes on a series of 200 patients evaluated for HIT and 100 non-thrombocytopenic hospitalized patients. The frequency of the His/His 131 phenotype was significantly increased (34.4%) in the 96 thrombocytopenic patients with HIT antibody compared to the 104 thrombocytopenic patients without HIT antibody and the 100 non-thrombocytopenic patients (approximately 19%). Thus, the Fc gamma RII phenotype regulates the in vitro activation response of normal platelets to HIT antibody and is a risk factor for the thrombocytopenia of HIT.

Surveillance of pediatric injury hospitalizations in Southern California

OBJECTIVES

This study was designed to determine the incidence and causes of injury hospitalizations/fatalities to children less than 15 years of age.

SETTING

Central Orange County, California.

DESIGN

Cases were identified through a population based hospital and coroner's office surveillance system.

SUBJECTS

The sample consisted of children 0-14 years of age who were residents of the study area and sustained an injury between 1 January 1991 and 31 December 1992 resulting in hospitalization or death.

RESULTS

Over the two year study period, 1361 children 0-14 years of age were hospitalized or died as a result of injury. This represents a crude annual injury rate of 318/100,000 children. Rates were highest for children less than 5 years--this age group sustained the highest rate for eight of nine specific causes of injury. Falls were the leading cause of hospitalizations for all ages. Pedestrian injuries were more common among children 1-4 years and 5-9 years, while bicycle injuries were more common among older children.

CONCLUSIONS

This study, one of the first population based studies in a Southern California urban/suburban community, found lower rates of injury hospitalization than studies conducted over a decade ago. These lower rates may reflect changes in hospitalization trends and/or injury prevention programs. Comparisons with more recent studies in inner city communities in the north east also show regional differences in rates and causes. Injury prevention efforts should particularly address the higher injury rates among children less than 5 years of age. This study also illustrates the need for regional and local data to guide injury control.

Interactions in nonnative and truncated forms of staphylococcal nuclease as indicated by mutational free energy changes

Several mixed disulfide variants of staphylococcal nuclease have been produced by disulfide bond formation between nuclease V23C and methane, ethane, 1-propane, 1-n-butane, and 1-n-pentane thiols. Although CD spectroscopy shows that the native state is largely unperturbed, the stability toward urea-induced unfolding is highly dependent on the nature of the group at this position, with the methyl disulfide protein being the most stable. The variant produced by modification with iodoacetic acid, however, gives a CD spectrum indicative of an unfolded polypeptide. Thiol-disulfide exchange equilibrium constants between nuclease V23C and 2-hydroxyethyl disulfide have been measured as a function of urea concentration. Because thiol-disulfide exchange and unfolding are thermodynamically linked, the effects of a mutation (disulfide exchange) can be partitioned between various conformational states. In the case of unmodified V23C and the 2-hydroxyethyl protein mixed disulfide, significant effects in the nonnative states of nuclease are observed. Truncated forms of staphylococcal nuclease are thought to be partially folded and may be good models for early folding intermediates. We have characterized a truncated form of nuclease comprised of residues 1-135 with a V23C mutation after chemical modification of the cysteine residue. High-resolution size-exclusion chromatography indicates that modification brings about significant changes in the Stokes radius of the protein, and CD spectroscopy indicates considerable differences in the amount of secondary structure present. Measurement of the disulfide exchange equilibrium constant between this truncated protein and 2-hydroxyethyl disulfide indicate significant interactions between position 23 and the rest of the protein when the urea concentration is lower than 1.5 M.(ABSTRACT TRUNCATED AT 250 WORDS)

Who carries passengers in the back of pickup trucks?

This study was designed to compare pickup truck drivers who carried passengers in the back with those who did not, with respect to driving behaviors, sociodemographic features, and issues related to use of the pickup truck. A computerized assisted telephone survey was conducted in Riverside County, California. One thousand ten motor vehicle drivers were interviewed with respect to demographics, restraint use, driving behaviors, and variables related to vehicle use. Thirty-six percent (364) of the households had a pickup truck driver. Pickup drivers were grouped into those who stated that they had carried occupants in the back (n = 119) and those who had not (n = 245). A higher proportion of 16- to 24-year-old and 35- to 44-year-old pickup truck drivers carried passengers in the back of pickup trucks. Those who carried occupants in the back were significantly more likely to be Hispanic, to be students, and to live in a household that included teenagers. They also had a larger mean household size. They were less likely to own the pickup and less likely to be the principal wage earners. They were more likely to report four high-risk driving behaviors. They also used the pickup truck for multiple purposes, i.e. recreation, work, school transportation, and daily transportation. In the multivariate analysis, the presence of teenagers in the household, three high-risk driving behaviors, and three indicators of pickup truck use were independently related to carrying passengers in the back of a pickup. Those who allowed passengers in the back were significantly less likely to agree with statements suggesting restriction of travel in the back. Finally, only 9% of the respondents who carried passengers in the back of a pickup reported that the pickup truck was the only vehicle available in the household.

Sensitivity of hospitals' E-coded data in identifying causes of children's violence-related injuries

E codes classify causes of injury as unintentional, intentional, and undetermined. E-coded discharge data from hospitals provide an opportunity to use this source of morbidity data for planning, implementation, and evaluation of injury and violence prevention activities. This study explores the extent to which E-coded data from hospitals identify injuries that result from violent acts. Cases were identified through a multihospital population-based surveillance system of pediatric injuries. Those cases with injury as a result of violence, designated by study criteria, were compared with those with E codes that were classified as intentional. The analysis indicated that 25 percent of injuries to children resulting from violence may not be accounted for through the use of E codes. The majority of the undetected injury cases resulting from violence involved child abuse. Researchers and other persons who design and conduct injury and violence prevention programs should be aware of the undercount and associated issues when using E-coded hospital discharge data for surveillance of injuries resulting from violence.

Clustering of the platelet Fc gamma receptor induces noncovalent association with the tyrosine kinase p72syk

The Fc receptor for IgG in platelets was identified as the integral membrane isoform encoded by the Fc gamma RIIA gene. Functional analysis of this molecule determined that activated Fc gamma RIIA is tyrosine phosphorylated and that activation induced the physical association with the protein tyrosine kinase p72syk. Other tyrosine-phosphorylated molecules also co-immunoadsorbed with the activated receptor. Tyrosine kinase activity co-immunoadsorbing with the platelet Fc gamma R was enhanced upon activation and specifically induced the phosphorylation, on tyrosine residues, of a physically associated 72-kDa protein. These data support a model of Fc gamma receptor-mediated platelet activation where signal is transduced through inducible association of the tyrosine kinase p72syk with the low affinity Fc gamma receptor. Thrombin, a potent platelet agonist, has been shown to up-regulate the activity of the tyrosine kinase p72syk in platelets. Consequently, our findings identify a second pathway by which p72syk is activated in platelets.

Ethnic variation in frequency of an allelic polymorphism of human Fc gamma RIIA determined with allele specific oligonucleotide probes

We have genotyped 53 individuals from three ethnic groups (Japanese, Chinese, Asian Indian) for an allotypic polymorphism of a widely expressed low affinity Fc receptor for IgG (Fc gamma RIIA). The method, requiring PCR amplification of genomic DNA and Southern analysis with allele specific oligonucleotide probes, detects a single nucleotide difference (G or A) at base 494 which results in an arginine (R) or histidine (H) at amino acid 131 of the Fc gamma RIIA protein. This polymorphism has been shown to determine the affinity of the receptor for hIgG2; Fc gamma RIIA-H131 has a high affinity for hIgG2, while Fc gamma RIIA-R131 binds hIgG2 weakly. We found that the Japanese and Chinese groups have an increased frequency of the H/H131 allotype (61 and 50% respectively) as compared to the Caucasian group (23%), in agreement with previously reported phenotype data. The genotype distribution of the Asian Indian group was not different from our Caucasian group. The shifts in frequency of the R131 and H131 alleles in different populations may have implications for disease susceptibility when the hIgG2 antibody isotype predominates.

Risk of pneumothorax not increased by obstructive lung disease in percutaneous needle biopsy

All computed tomography directed lung biopsies from 1987 to 1991 were retrospectively reviewed. Pneumothorax occurred in 33 of 93 (35 percent) patients; 16 of these patients or 17 percent overall required a chest tube. No correlation was found between the presence of obstructive lung disease and the incidence of pneumothorax. Patients with smaller lesions were more likely to incur a pneumothorax. No relation was found with central or peripheral location, patient's age, or the operator and the incidence of pneumothorax. Patients with a pneumothorax were more likely to require a chest tube if they had obstructive lung disease.

Shock-induced Analgesia in the cockroach (Periplaneta americana)

For 3 consecutive days cockroaches (Periplaneta americana) received escapable, inescapable, or no shock in an escape task. 24 hr. later minimum shock which initiated movement was identified. Reliably higher shocks were needed to initiate movement in the inescapably shocked roaches. In a second experiment the analgesia induced by inescapable shock was blocked by the opiate antagonist naloxone. The results are discussed in relation to the escape deficit and analgesia commonly seen following exposure to inescapable shock in a variety of species.

Plasma endotoxin concentration after an intraperitoneal injection of endotoxin in fed and fasted suckling rats

In the adult host response to endotoxin (lipopolysaccharide (LPS)) is dose-related. An intraperitoneal injection is commonly used for LPS administration in small animals. However, plasma endotoxin concentration following an intraperitoneal bolus injection of LPS is not well known. This study was performed to evaluate plasma endotoxin concentration following a bolus intraperitoneal injection of LPS in both fed and 24 h fasted 10 day old rats. Plasma endotoxin concentration increased in a dose-dependent manner after LPS injection (.03 or .1 mg/kg Salmonella enteritidis LPS) in both fed and fasted rats. Plasma endotoxin concentrations were higher (p < .05) in fed than fasted rats. A high dose of LPS (.1 mg/kg) induced 95 and 40% mortality in fed and fasted rats, respectively. A low dose of LPS (.03 mg/kg) induced 26.7% mortality in fed rats but no mortality in fasted rats. The hematocrit was significantly lower in fed than fasted rats. Plasma endotoxin inactivation was similar in fed and fasted rats. Host response appears to be related to plasma endotoxin concentration.

Clustering of the high affinity Fc receptor for immunoglobulin G (Fc gamma RI) results in phosphorylation of its associated gamma-chain

We are investigating the role of gamma-chain in functions mediated by the high affinity Fc receptor for IgG (Fc gamma RI). In a previous study, we found that gamma-chain, which is a member of the family of zeta-chain proteins, associates with Fc gamma RI. Here we show that clustering of Fc gamma RI leads to a rapid and transient tyrosine phosphorylation of gamma-chain in U937 cells. The response was limited to Fc gamma RI activation, and no phosphorylation of gamma-chain was observed after cross-linking of monoclonal antibodies to other surface receptors on these cells. The gamma-chain phosphorylated after Fc gamma RI clustering was the gamma-chain associated with the receptor. We also identified Syk as one of the kinases associated with the receptor complex. Upon Fc gamma RI activation, Syk, but not ZAP-70, was phosphorylated, and reimmunoadsorption experiments of phosphoproteins from immune complex in vitro kinase assays indicated that Syk is part of the activated gamma-chain-Fc gamma RI complex. These results suggest that gamma-chain links Fc gamma RI to intracellular transduction pathways.

Differences in child pedestrian injury events by location

OBJECTIVE

To compare child pedestrian injury events occurring in driveways and parking lots and at midblock and intersections with respect to characteristics and activity of the child, injury outcome measures, and characteristics of the vehicle and roadway.

DESIGN

Descriptive case series.

SETTING

Data were obtained from a multihospital/coroner monitoring system, during 2 years in an urban county, by record review and interviews.

PATIENTS

The sample consisted of 345 pedestrians 0 through 14 years of age treated for injuries at one of the participating facilities.

RESULTS

Eleven percent were injured in driveways, 8% in parking lots, 53% at midblock, and 28% at intersections. Median age was 2 years for driveways, 4 years for parking lots, 6 years for midblock, and 10 years for intersection. Events in driveways and parking lots had significantly more vehicles backing up, fewer automobiles, and more pedestrians with adults. Events at intersections occurred more often on streets with more than two lanes, with speed limits > 25 mph, and with moderate or heavy traffic than events at midblock. Sixteen percent of those injured in driveways and parking lots sustained head injury of moderate or greater severity versus 35% injured in the street.

CONCLUSIONS

Interventions to prevent child pedestrian injuries must consider normal child behavior and driver awareness as it relates to location of the events. Driveway events involve small children, larger vehicles, and backing up. Midblock events involve children too young to cross even quiet residential streets safely. Traffic controls and safe street crossing skills are measures to consider for intersection events involving older children.

The VA computerized Patient Record--a first look

In support of its in-house DHCP Physician Order Entry/Results Reporting application, the VA is developing the first edition of a Computerized Patient Record. The system will feature a physician-oriented interface with real time, expert system-based order checking, a controlled vocabulary, a longitudinal repository of patient data, HL7 messaging support, a clinical reminder and warning system, and full integration with existing VA applications including lab, pharmacy, A/D/T, radiology, dietetics, surgery, vitals, allergy tracking, discharge summary, problem list, progress notes, consults, and online physician order entry.

Macrophage treatment in suckling rat endotoxic shock

Gram-negative sepsis/septic shock in the human newborn continues to be a severe medical problem because of significant mortality and morbidity. Since macrophages detoxify endotoxin, a decreased number of macrophages may contribute to the newborn's sensitivity to endotoxin. In this study, peritoneal macrophages were used for the treatment of endotoxic shock in 10-day-old rats, and 24-hr mortality, plasma glucose, and lactate concentrations were monitored. Peritoneal macrophages were harvested from adult or 10-day-old rats. Caseinate-stimulated macrophages from adult and 10-day-old rats significantly decreased the mortality of 10-day-old rat endotoxic shock from 90% to 37.5% and 44.4%, respectively. Resident macrophages from adult and 10-day-old rats also decreased the mortality from 90% to 12.5% and 45.4%, respectively. Peritoneal macrophages from adult rats significantly ameliorated hypoglycemia during endotoxic shock in a dose-dependent manner. Macrophage treatment decreased plasma endotoxin concentration (P < 0.05). Macrophage treatment was important for host defense.

An association of pulmonary hypoplasia with unilateral agenesis of the diaphragm

During a period of 5 years, 33 newborns with congenital diaphragmatic hernia were treated. Three groups presenting with respiratory distress in the delivery room were identified. These included 8 newborns with agenesis (group 1) and 4 newborns with nonagenesis (group 2), all of whom died. There were 19 nonagenesis survivors (group 3), giving an overall survival rate of 61%. Two newborns who presented beyond 6 hours of life were excluded. No one specific arterial blood gas value or ventilation parameter obtained preoperatively could predict survival. Postmortem right and left lung weights, lung/body weight ratio, and radial alveolar counts demonstrate that agenesis is a unique subgroup with profound pulmonary hypoplasia and a dismal prognosis.

Cellular localization of CD4 in the human placenta. Implications for maternal-to-fetal transmission of HIV

CD4 is a 55-kDa glycoprotein that serves as an important cellular differentiation Ag and cell signaling protein on T lymphocytes, as well as a principal receptor for HIV-1 on a variety of cell types including lymphocytes. CD4 receptor expression in syncytiotrophoblasts, the principal cellular barrier in the human placenta, has not been clearly defined. Knowledge concerning the expression of the CD4 receptor on placental trophoblasts is important to define potential mechanisms of transmission of the virus between maternal blood and fetal tissues. Both mature and immature placenta (n = 10) were examined using an avidin D-based immunohistochemical procedure that permits clear morphologic distinction of cell types in placental sections. Syncytiotrophoblasts were defined using anti-cytokeratin mAb (AE1/3), whereas endothelial cells in placental villi were distinctly identified using a mAb directed to CD31. Placental Hofbauer cells (macrophages) and other leukocytes were identified by mAb staining of leukocyte common Ag (CD45). CD4 expression (identified by staining with three separate anti-CD4 mAb) was exclusively localized using this immunohistochemical method to leukocytes in placental villi (e.g., Hofbauer cells); however, no CD4 staining was evident in syncytiotrophoblasts, cytotrophoblasts, or villus endothelial cells. Furthermore, immunoaffinity-purified trophoblasts were negative for CD4 receptor expression. CD4 RNA was not identified in purified trophoblasts using both Northern blot assay and a sensitive polymerase chain reaction method to identify CD4 RNA. In addition, time course studies of purified trophoblasts immediately after purification and at 24, 48, and 72 h in culture indicated that CD4 RNA was not present as a transient, but labile transcript in trophoblasts. These data indicate that the transmission of HIV-1 across syncytiotrophoblasts may occur by mechanisms other than by binding the CD4 receptor and that tissue leukocytes (in particular Hofbauer cells) are likely the principal CD4+ cellular target of HIV-1 in the placenta.

Cellular localization of CD4 in the human placenta. Implications for maternal-to-fetal transmission of HIV

CD4 is a 55-kDa glycoprotein that serves as an important cellular differentiation Ag and cell signaling protein on T lymphocytes, as well as a principal receptor for HIV-1 on a variety of cell types including lymphocytes. CD4 receptor expression in syncytiotrophoblasts, the principal cellular barrier in the human placenta, has not been clearly defined. Knowledge concerning the expression of the CD4 receptor on placental trophoblasts is important to define potential mechanisms of transmission of the virus between maternal blood and fetal tissues. Both mature and immature placenta (n = 10) were examined using an avidin D-based immunohistochemical procedure that permits clear morphologic distinction of cell types in placental sections. Syncytiotrophoblasts were defined using anti-cytokeratin mAb (AE1/3), whereas endothelial cells in placental villi were distinctly identified using a mAb directed to CD31. Placental Hofbauer cells (macrophages) and other leukocytes were identified by mAb staining of leukocyte common Ag (CD45). CD4 expression (identified by staining with three separate anti-CD4 mAb) was exclusively localized using this immunohistochemical method to leukocytes in placental villi (e.g., Hofbauer cells); however, no CD4 staining was evident in syncytiotrophoblasts, cytotrophoblasts, or villus endothelial cells. Furthermore, immunoaffinity-purified trophoblasts were negative for CD4 receptor expression. CD4 RNA was not identified in purified trophoblasts using both Northern blot assay and a sensitive polymerase chain reaction method to identify CD4 RNA. In addition, time course studies of purified trophoblasts immediately after purification and at 24, 48, and 72 h in culture indicated that CD4 RNA was not present as a transient, but labile transcript in trophoblasts. These data indicate that the transmission of HIV-1 across syncytiotrophoblasts may occur by mechanisms other than by binding the CD4 receptor and that tissue leukocytes (in particular Hofbauer cells) are likely the principal CD4+ cellular target of HIV-1 in the placenta.