The US Department of Veterans Affairs Science and Health Initiative to Combat Infectious and Emerging Life-Threatening Diseases (VA SHIELD): A Biorepository Addressing National Health Threats

Background

The coronavirus disease 2019 (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has demonstrated the need to share data and biospecimens broadly to optimize clinical outcomes for US military Veterans.

Methods

In response, the Veterans Health Administration established VA SHIELD (Science and Health Initiative to Combat Infectious and Emerging Life-threatening Diseases), a comprehensive biorepository of specimens and clinical data from affected Veterans to advance research and public health surveillance and to improve diagnostic and therapeutic capabilities.

Results

VA SHIELD now comprises 12 sites collecting de-identified biospecimens from US Veterans affected by SARS-CoV-2. In addition, 2 biorepository sites, a data processing center, and a coordinating center have been established under the direction of the Veterans Affairs Office of Research and Development. Phase 1 of VA SHIELD comprises 34 157 samples. Of these, 83.8% had positive tests for SARS-CoV-2, with the remainder serving as contemporaneous controls. The samples include nasopharyngeal swabs (57.9%), plasma (27.9%), and sera (12.5%). The associated clinical and demographic information available permits the evaluation of biological data in the context of patient demographics, clinical experience and management, vaccinations, and comorbidities.

Conclusions

VA SHIELD is representative of US national diversity with a significant potential to impact national healthcare. VA SHIELD will support future projects designed to better understand SARS-CoV-2 and other emergent healthcare crises. To the extent possible, VA SHIELD will facilitate the discovery of diagnostics and therapeutics intended to diminish COVID-19 morbidity and mortality and to reduce the impact of new emerging threats to the health of US Veterans and populations worldwide.

Effectiveness of mRNA COVID-19 vaccines against Omicron and Delta variants in a matched test-negative case-control study among US veterans

OBJECTIVE

To estimate the effectiveness of messenger RNA (mRNA) booster doses during the period of Delta and Omicron variant dominance.

DESIGN

We conducted a matched test-negative case-control study to estimate the vaccine effectiveness (VE) of three and two doses of mRNA vaccines against infection (regardless of symptoms) and against COVID-19-related hospitalisation and death.

SETTING

Veterans Health Administration.

PARTICIPANTS

We used electronic health record data from 114 640 veterans who had a SARS-CoV-2 test during November 2021-January 2022. Patients were largely 65 years or older (52%), male (88%) and non-Hispanic white (59%).

MAIN OUTCOME MEASURES

First positive result for a SARS-CoV-2 PCR or antigen test.

RESULTS

Against infection, booster doses had higher estimated VE (64%, 95% CI 63 to 65) than two-dose vaccination (12%, 95% CI 10 to 15) during the Omicron period. For the Delta period, the VE against infection was 90% (95% CI 88 to 92) among boosted vaccinees, higher than the VE among two-dose vaccinees (54%, 95% CI 50 to 57). Against hospitalisation, booster dose VE was 89% (95% CI 88 to 91) during Omicron and 94% (95% CI 90 to 96) during Delta; two-dose VE was 63% (95% CI 58 to 67) during Omicron and 75% (95% CI 69 to 80) during Delta. Against death, the VE with a booster dose was 94% (95% CI 90 to 96) during Omicron and 96% (95% CI 87 to 99) during Delta.

CONCLUSIONS

Among an older, mostly male, population with comorbidities, we found that an mRNA vaccine booster was highly effective against infection, hospitalisation and death. Although the effectiveness of booster vaccination against infection was moderately higher against Delta than against the Omicron SARS-CoV-2 variant, effectiveness against severe disease and death was similarly high against both variants.

Risk Factors for Acute Gastroenteritis Among Patients Hospitalized in 5 Veterans Affairs Medical Centers, 2016-2019

BACKGROUND

In the United States, ∼179 million acute gastroenteritis (AGE) episodes occur annually. We aimed to identify risk factors for all-cause AGE, norovirus-associated vs non-norovirus AGE, and severe vs mild/moderate AGE among hospitalized adults.

METHODS

We enrolled 1029 AGE cases and 624 non-AGE controls from December 1, 2016, to November 30, 2019, at 5 Veterans Affairs Medical Centers. Patient interviews and medical chart abstractions were conducted, and participant stool samples were tested using the BioFire Gastrointestinal Panel. Severe AGE was defined as a modified Vesikari score of ≥11. Multivariate logistic regression was performed to assess associations between potential risk factors and outcomes; univariate analysis was conducted for norovirus-associated AGE due to limited sample size.

RESULTS

Among 1029 AGE cases, 551 (54%) had severe AGE and 44 (4%) were norovirus positive. Risk factors for all-cause AGE included immunosuppressive therapy (adjusted odds ratio [aOR], 5.6; 95% CI, 2.7-11.7), HIV infection (aOR, 3.9; 95% CI, 1.8-8.5), severe renal disease (aOR, 3.1; 95% CI, 1.8-5.2), and household contact with a person with AGE (aOR, 2.9; 95% CI, 1.3-6.7). Household (OR, 4.4; 95% CI, 1.6-12.0) and non-household contact (OR, 5.0; 95% CI, 2.2-11.5) with AGE was associated with norovirus-associated AGE. Norovirus positivity (aOR, 3.4; 95% CI, 1.3-8.8) was significantly associated with severe AGE.

CONCLUSIONS

Patients with immunosuppressive therapy, HIV, and severe renal disease should be monitored for AGE and may benefit from targeted public health messaging regarding AGE prevention. These results may also direct future public health interventions, such as norovirus vaccines, to specific high-risk populations.

Association of Secretor Status and Recent Norovirus Infection With Gut Microbiome Diversity Metrics in a Veterans Affairs Population

Norovirus infection causing acute gastroenteritis could lead to adverse effects on the gut microbiome. We assessed the association of microbiome diversity with norovirus infection and secretor status in patients from Veterans Affairs medical centers. Alpha diversity metrics were lower among patients with acute gastroenteritis but were similar for other comparisons.

Comparative Effectiveness and Antibody Responses to Moderna and Pfizer-BioNTech COVID-19 Vaccines among Hospitalized Veterans - Five Veterans Affairs Medical Centers, United States, February 1-September 30, 2021

The mRNA COVID-19 vaccines (Moderna and Pfizer-BioNTech) provide strong protection against severe COVID-19, including hospitalization, for at least several months after receipt of the second dose (1,2). However, studies examining immune responses and differences in protection against COVID-19-associated hospitalization in real-world settings, including by vaccine product, are limited. To understand how vaccine effectiveness (VE) might change with time, CDC and collaborators assessed the comparative effectiveness of Moderna and Pfizer-BioNTech vaccines in preventing COVID-19-associated hospitalization at two periods (14-119 days and ≥120 days) after receipt of the second vaccine dose among 1,896 U.S. veterans at five Veterans Affairs medical centers (VAMCs) during February 1-September 30, 2021. Among 234 U.S. veterans fully vaccinated with an mRNA COVID-19 vaccine and without evidence of current or prior SARS-CoV-2 infection, serum antibody levels (anti-spike immunoglobulin G [IgG] and anti-receptor binding domain [RBD] IgG) to SARS-CoV-2 were also compared. Adjusted VE 14-119 days following second Moderna vaccine dose was 89.6% (95% CI = 80.1%-94.5%) and after the second Pfizer-BioNTech dose was 86.0% (95% CI = 77.6%-91.3%); at ≥120 days VE was 86.1% (95% CI = 77.7%-91.3%) for Moderna and 75.1% (95% CI = 64.6%-82.4%) for Pfizer-BioNTech. Antibody levels were significantly higher among Moderna recipients than Pfizer-BioNTech recipients across all age groups and periods since vaccination; however, antibody levels among recipients of both products declined between 14-119 days and ≥120 days. These findings from a cohort of older, hospitalized veterans with high prevalences of underlying conditions suggest the importance of booster doses to help maintain long-term protection against severe COVID-19.^†.

Adapting the Surveillance Platform for Enteric and Respiratory Infectious Organisms at United States Veterans Affairs Medical Centers (SUPERNOVA) for COVID-19 Among Hospitalized Adults: Surveillance Protocol

Introduction: Early in the COVID-19 pandemic, the Centers for Disease Control and Prevention (CDC) rapidly initiated COVID-19 surveillance by leveraging existing hospital networks to assess disease burden among hospitalized inpatients and inform prevention efforts.

Materials and Methods: The Surveillance Platform for Enteric and Respiratory Infectious Organisms at Veterans Affairs Medical Centers (SUPERNOVA) is a network of five United States Veterans Affairs Medical Centers which serves nearly 400,000 Veterans annually and conducts laboratory-based passive and active monitoring for pathogens associated with acute gastroenteritis and acute respiratory illness among hospitalized Veterans. This paper presents surveillance methods for adapting the SUPERNOVA surveillance platform to prospectively evaluate COVID-19 epidemiology during a public health emergency, including detecting, characterizing, and monitoring patients with and without COVID-19 beginning in March 2020. To allow for case-control analyses, patients with COVID-19 and patients with non-COVID-19 acute respiratory illness were included.

Results: SUPERNOVA included 1,235 participants with COVID-19 and 707 participants with other acute respiratory illnesses hospitalized during February through December 2020. Most participants were male (93.1%), with a median age of 70 years, and 45.8% non-Hispanic Black and 32.6% non-Hispanic White. Among those with COVID-19, 28.2% were transferred to an intensive care unit, 9.4% received invasive mechanical ventilation, and 13.9% died. Compared with controls, after adjusting for age, sex, and race/ethnicity, COVID-19 case-patients had significantly higher risk of mortality, respiratory failure, and invasive mechanical ventilation, and longer hospital stays.

Discussion: Strengths of the SUPERNOVA platform for COVID-19 surveillance include the ability to collect and integrate multiple types of data, including clinical and illness outcome information, and SARS-CoV-2 laboratory test results from respiratory and serum specimens. Analysis of data from this platform also enables formal comparisons of participants with and without COVID-19. Surveillance data collected during a public health emergency from this key U.S. population of Veterans will be useful for epidemiologic investigations of COVID-19 spectrum of disease, underlying medical conditions, virus variants, and vaccine effectiveness, according to public health priorities and needs.

Complexing amphotericin B with gold nanoparticles improves fungal clearance from the brains of mice infected with Cryptococcal neoformans

Amphotericin B (AmB) is used to treat cryptococcal meningoencephalitis. However, the mortality rate remains high. Higher doses of AmB in deoxycholate buffer (AmBd) are toxic to human red blood cells (hRBC) and have no effect on brain organism load in mice. Here we show that while AmBd lysed 96% of hRBC, AmB complexed with gold nanoparticles (AuNP-SA-AmB) lysed only 27% of hRBC. In vitro growth of C. neoformans was inhibited by 0.25 μg/ml AmBd and 0.04 μg/ml of AuNP-SA-AmB. In mice infected with C. neoformans, five daily treatments with AuNP-SA-AmB containing 0.25 mg/kg AmB significantly lowered the fungal burden in the brain tissue compared to either untreated or treatment with 0.25 mg/kg of AmBd. When a single dose of AmBd was injected intravenously into BALB/c mice, 81.61% of AmB cleared in the α-phase and 18.39% cleared in the β-phase at a rate of 0.34% per hour. In contrast, when AuNP-SA-AmB was injected, 49.19% of AmB cleared in the α-phase and 50.81% of AmB cleared in the β-phase at a rate of 0.27% per hour. These results suggest that AmB complexed with gold nanoparticles is less toxic to hRBC, is more effective against C. neoformans and persists longer in blood when injected into mice resulting in more effective clearing of C. neoformans from the brain tissue.

LAY SUMMARY

Amphotericin B (AmB) was complexed with gold nanoparticles (AuNP-SA-AmB) to improve brain delivery. AuNP-SA-AmB was more effective than AmB alone in clearing of Cryptococcus neoformans from the brain tissue of infected mice. This may be due to longer plasma half-life of AmB as AuNP-SA-AmB.

Effectiveness of COVID-19 mRNA Vaccines Against COVID-19-Associated Hospitalization - Five Veterans Affairs Medical Centers, United States, February 1-August 6, 2021

COVID-19 mRNA vaccines (Pfizer-BioNTech and Moderna) have been shown to be highly protective against COVID-19-associated hospitalizations (1-3). Data are limited on the level of protection against hospitalization among disproportionately affected populations in the United States, particularly during periods in which the B.1.617.2 (Delta) variant of SARS-CoV-2, the virus that causes COVID-19, predominates (2). U.S. veterans are older, more racially diverse, and have higher prevalences of underlying medical conditions than persons in the general U.S. population (2,4). CDC assessed the effectiveness of mRNA vaccines against COVID-19-associated hospitalization among 1,175 U.S. veterans aged ≥18 years hospitalized at five Veterans Affairs Medical Centers (VAMCs) during February 1-August 6, 2021. Among these hospitalized persons, 1,093 (93.0%) were men, the median age was 68 years, 574 (48.9%) were non-Hispanic Black (Black), 475 were non-Hispanic White (White), and 522 (44.4%) had a Charlson comorbidity index score of ≥3 (5). Overall adjusted vaccine effectiveness against COVID-19-associated hospitalization was 86.8% (95% confidence interval [CI] = 80.4%-91.1%) and was similar before (February 1-June 30) and during (July 1-August 6) SARS-CoV-2 Delta variant predominance (84.1% versus 89.3%, respectively). Vaccine effectiveness was 79.8% (95% CI = 67.7%-87.4%) among adults aged ≥65 years and 95.1% (95% CI = 89.1%-97.8%) among those aged 18-64 years. COVID-19 mRNA vaccines are highly effective in preventing COVID-19-associated hospitalization in this older, racially diverse population of predominately male U.S. veterans. Additional evaluations of vaccine effectiveness among various age groups are warranted. To prevent COVID-19-related hospitalizations, all eligible persons should receive COVID-19 vaccination.

Using Serologic Testing to Assess the Effectiveness of Outbreak Control Efforts, Serial Polymerase Chain Reaction Testing, and Cohorting of Positive Severe Acute Respiratory Syndrome Coronavirus 2 Patients in a Skilled Nursing Facility

We characterized serology following a nursing home outbreak where residents were serially tested by RT-PCR and positive residents were cohorted. When tested 46-76 days later, 24/26 RT-PCR-positive residents were seropositive; none of the 124 RT-PCR-negative residents had confirmed seropositivity, supporting serial SARS-CoV-2 RT-PCR testing and cohorting in nursing homes.

Trends in Incidence of Norovirus-associated Acute Gastroenteritis in 4 Veterans Affairs Medical Center Populations in the United States, 2011-2015

BACKGROUND

Norovirus is an important cause of epidemic acute gastroenteritis (AGE), yet the burden of endemic disease in adults has not been well documented. We estimated the prevalence and incidence of outpatient and community-acquired inpatient norovirus AGE at 4 Veterans Affairs Medical Centers (VAMC) (Atlanta, Georgia; Bronx, New York; Houston, Texas; and Los Angeles, California) and examined trends over 4 surveillance years.

METHODS

From November 2011 to September 2015, stool specimens collected within 7 days of AGE symptom onset for clinician-requested diagnostic testing were tested for norovirus, and positive samples were genotyped. Incidence was calculated by multiplying norovirus prevalence among tested specimens by AGE-coded outpatient encounters and inpatient discharges, and dividing by the number of unique patients served.

RESULTS

Of 1603 stool specimens, 6% tested were positive for norovirus; GII.4 viruses (GII.4 New Orleans [17%] and GII.4 Sydney [47%]) were the most common genotypes. Overall prevalence and outpatient and inpatient community-acquired incidence followed a seasonal pattern, with higher median rates during November-April (9.2%, 376/100 000, and 45/100 000, respectively) compared to May-October (3.0%, 131/100 000, and 13/100 000, respectively). An alternate-year pattern was also detected, with highest peak prevalence and outpatient and inpatient community-acquired norovirus incidence rates in the first and third years of surveillance (14%-25%, 349-613/100 000, and 43-46/100 000, respectively).

CONCLUSIONS

This multiyear analysis of laboratory-confirmed AGE surveillance from 4 VAMCs demonstrates dynamic intra- and interannual variability in prevalence and incidence of outpatient and inpatient community-acquired norovirus in US Veterans, highlighting the burden of norovirus disease in this adult population.

512. Kinetics of SARS-CoV-2 IgG responses among hospitalized patients with COVID-19

Background

The kinetics of antibody responses to SARS-CoV-2 infection are not fully understood. We analyzed IgG responses to the SARS-CoV-2 Spike protein receptor binding domain (RBD) in COVID-19 patients admitted to VA Greater Los Angeles (VAGLA) and correlated with clinical outcomes.

Methods

Serially admitted patients from March 20-May 10, 2020 with at least one available residual serum specimen were included in this analysis. Serum samples selected for analysis included first, last, and intermediaries spaced ≥ 5 days apart, as available. Anti-RBD IgG was detected with an enzyme immunoassay (EIA) using recombinant RBD protein. Serum from an uninfected individual collected April 2019 was used as control. The average optical density of the control in triplicate plus 3 standard deviations was considered the threshold positive/negative value. The highest dilution above the threshold value was considered the IgG titer. Clinical groups were defined as asymptomatic, moderate/severe (no ICU) or critical (mechanical ventilation, cytokine storm and/or death).

Results

Of the 43 consecutive patients admitted to VAGLA with COVID-19 in this analysis, 40 developed detectable RBD IgG responses with maximum inverse titers (MIT) ranging 100-819,200, geometric mean 12,152. Five patients remained asymptomatic but had positive EIAs with median MIT 3200 (IQR 800–3200). Twenty-five had moderate-severe illness with median MIT 25600 (IQR 6400–102400). Ten patients with critical disease had median MIT 38400 (IQR 8800–51200). The median time to positive IgG was 10 days for asymptomatic (IQR 10,10), 4 days for moderate-severe (IQR 3,15), and 7 days for critical (IQR 3.5,14.5). The figure depicts RBD IgG titers over time after onset of symptoms. Asymptomatic patients had a more gradual rate of increase and lower peak titers, while critical patients had the fastest rate of rise and the highest peak titers. Of the 21 patients with samples > 30 days after symptom onset (range 31–67 days), there was no evidence for decrease in anti-RBD IgG.

Kinetics of IgG to SARS-CoV-2 receptor binding domain by clinical severity

Conclusion

Following infection with SARS-CoV-2, disease severity correlates with both the rate of increase and peak in antibody titers. Anti-RBD IgG titers did not decrease over the observation period.

Disclosures

All Authors: No reported disclosures

70. Lack of SARS-CoV-2 Antibody Seroconversion After Prompt Identification and Cohorting of Sentinel sars-cov-2-positive Residents in a Skilled Nursing Facility

Background

Despite numerous outbreaks, antibody responses to SARS-CoV-2 in residents of skilled nursing facilities (SNF) are not well described. We reviewed serological test results in a cohort of SNF residents who had been repetitively screened for SARS-CoV-2 infection by nasopharyngeal swab PCR.

Methods

In late March 2019, we identified symptomatic SARS-CoV-2 PCR positive residents at a SNF. In response, all remaining SNF patients were serially screened, and all SARS-CoV-2 PCR positive patients were transferred to the acute care hospital or cohorted in a separate COVID Recovery Unit (CRU) in the SNF. In early June, all SNF residents (SARS-CoV-2 PCR positive and negative) underwent serologic testing for SARS-CoV-2 Spike (S1/S2) IgG (DiaSorin). DiaSorin IgG-positive results for patients that were SARS-CoV-2 PCR-negative were reflexed to nucleocapsid IgG (Abbott). Antibody testing occurred a median of 69 days (63–70 IQR) after PCR positivity.

Results

Nineteen SARS-CoV-2 PCR positive residents were identified from the outbreak and an additional 9 were transferred from the acute care hospital to the CRU; 1 died and 1 received convalescent plasma leaving 26 SARS-CoV-2 PCR positive residents, including 6 who were asymptomatic, that were eligible for serologic testing. Twenty-four of the 26 were positive for IgG by the DiaSorin assay; one seronegative resident was one of the asymptomatic residents. There were an additional 121 residents in the SNF whose SARS-CoV-2 PCR was negative at least once. Among these 121 SNF residents with negative SARS-CoV-2 RT-PCR, all but two were seronegative by the Diasorin assay. The two seropositive residents had no nucleocapsid antibodies when reflex tested by the Abbott assay.

Conclusion

In a limited sample of SNF residents with SARS-CoV-2 PCR positivity, the sensitivity of the Diasorin assay was 92% (24/26) and the specificity was 98% (119/121). None of the residents with negative SARS-CoV-2 PCR had confirmed positive antibody results using reflex testing (DiaSorin/Abbott). Despite high risk exposure in congregate living facilities, we found no evidence of additional SARS-CoV-2 exposure, reinforcing the importance of serial surveillance SARS-CoV-2 testing and early cohorting in SNF settings.

Disclosures

All Authors: No reported disclosures

Incidence, etiology, and severity of acute gastroenteritis among prospectively enrolled patients in 4 Veterans Affairs hospitals and outpatient centers, 2016-18

BACKGROUND

Acute gastroenteritis (AGE) burden, etiology, and severity in adults is not well-characterized. We implemented a multisite AGE surveillance platform in 4 Veterans Affairs Medical Centers (Atlanta, Bronx, Houston and Los Angeles), collectively serving >320,000 patients annually.

METHODS

From July 1, 2016-June 30, 2018, we actively identified AGE inpatient cases and non-AGE inpatient controls through prospective screening of admitted patients and passively identified outpatient cases through stool samples submitted for clinical diagnostics. We abstracted medical charts and tested stool samples for 22 pathogens via multiplex gastrointestinal PCR panel followed by genotyping of norovirus- and rotavirus-positive samples. We determined pathogen-specific prevalence, incidence, and modified Vesikari severity scores.

RESULTS

We enrolled 724 inpatient cases, 394 controls, and 506 outpatient cases. Clostridioides difficile and norovirus were most frequently detected among inpatients (cases vs controls: C. difficile, 18.8% vs 8.4%; norovirus, 5.1% vs 1.5%; p<0.01 for both) and outpatients (norovirus: 10.7%; C. difficile: 10.5%). Incidence per 100,000 population was highest among outpatients (AGE: 2715; C. difficile: 285; norovirus: 291) and inpatients ≥65 years old (AGE: 459; C. difficile: 91; norovirus: 26). Clinical severity scores were highest for inpatient norovirus, rotavirus, and Shigella/EIEC cases. Overall, 12% of AGE inpatient cases had ICU stays and 2% died; 3 deaths were associated with C. difficile and 1 with norovirus. C. difficile and norovirus were detected year-round with a fall/winter predominance.

CONCLUSIONS

C. difficile and norovirus were leading AGE pathogens in outpatient and hospitalized US Veterans, resulting in severe disease. Clinicians should remain vigilant for bacterial and viral causes of AGE year-round.

Validation of Acute Gastroenteritis-related International Classification of Diseases, Clinical Modification Codes in Pediatric and Adult US Populations

International Classification of Diseases diagnostic codes are used to estimate acute gastroenteritis (AGE) disease burden. We validated AGE-related codes in pediatric and adult populations using 2 multiregional active surveillance platforms. The sensitivity of AGE codes was similar (54% and 58%) in both populations and increased with addition of vomiting-specific codes.

652. What Is Blood Got to Do with It? Genetic Susceptibility to Norovirus and Rotavirus Infection: Results From the SUPERNOVA Network

Background

Histo-blood group antigens (HBGAs), whose expression is controlled in part by fucosyltransferase 2 (FUT2) and 3 (FUT3) genes, serve as receptors for norovirus and rotavirus. Individuals without functional FUT2 (nonsecretors) or FUT3 (Lewis-negative) genes may have decreased susceptibility to norovirus and rotavirus infections. As the prevalence of secretor and Lewis status can vary by race and ethnicity, we assessed this association in a US Veteran population.

Methods

Stool and saliva specimens were collected from acute gastroenteritis (AGE) cases and age- and time-matched controls through a multisite, active surveillance platform at four Veterans Affairs hospitals (Atlanta, Bronx, Houston, Los Angeles). Stool specimens were tested with the FilmArray Gastrointestinal Panel; norovirus and rotavirus positive specimens were genotyped. Saliva specimens were analyzed for HBGA expression by EIA using glycan-specific monoclonal antibodies and lectins. Chi-squared and Fisher’s exact tests were conducted to evaluate associations between secretor and Lewis status and infection with norovirus or rotavirus.

Results

From November 4, 2015–December 30, 2017, 670 AGE cases and 319 controls provided both stool and saliva specimens. Norovirus (21 GII.4 Sydney, 13 GII non-4, 7 GI, 10 untyped) and rotavirus (13 G12P[8], 1 G2P[4], 1 untyped) positive cases were more likely to be secretor positive (90% and 100%, respectively) compared with controls (76%) (P = 0.03 for both). Infections with GII.4 Sydney norovirus (P < 0.01) and G12P[8] rotavirus (P < 0.05) were significantly associated with secretor status. This association was not observed with other norovirus or rotavirus genotypes. No association was observed between Lewis status, race, or ethnicity and infection with norovirus or rotavirus.

Conclusion

Norovirus and rotavirus infections among a US Veteran population were associated with secretor status in a genotype-dependent manner, and with GII.4 Sydney norovirus and G12P[8] rotavirus, the most common strains. These associations are consistent with previously reported results, and suggest that the efficacy of interventions, such as vaccines, should include consideration of secretor status and predominantly circulating virus strains.

Disclosures

R. L. Atmar, Takeda Vaccines, Inc.: Investigator, Research grant. V. C. Marconi, ViiV: Investigator, Research support and Salary. Gilead: Investigator, Research support. Bayer: Investigator, Research support.

Evaluation of RIDA®GENE norovirus GI/GII real time RT-PCR using stool specimens collected from children and adults with acute gastroenteritis

BACKGROUND

Norovirus is the leading cause of epidemic and sporadic acute gastroenteritis (AGE) in the United States. Widespread prevalence necessitates implementation of accurate norovirus detection assays in clinical diagnostic laboratories.

OBJECTIVE

To evaluate RIDA^®GENE norovirus GI/GII real-time RT-PCR assay (RGN RT-PCR) using stool samples from patients with sporadic AGE.

STUDY DESIGN

Patients between 14 days to 101 years of age with symptoms of AGE were enrolled prospectively at four sites across the United States during 2014-2015. Stool specimens were screened for the presence of norovirus RNA by the RGN RT-PCR assay. Results were compared with a reference method that included conventional RT-PCR and sequencing of a partial region of the 5'end of the norovirus ORF2 gene.

RESULTS

A total of 259 (36.0%) of 719 specimens tested positive for norovirus by the reference method. The RGN RT-PCR assay detected norovirus in 244 (94%) of these 259 norovirus positive specimens. The sensitivity and specificity (95% confidence interval) of the RGN RT-PCR assay for detecting norovirus genogroup (G) I was 82.8% (63.5-93.5) and 99.1% (98.0-99.6) and for GII was 94.8% (90.8-97.2) and 98.6% (96.9-99.4), respectively. Seven specimens tested positive by the RGN-RT PCR that were negative by the reference method. The fifteen false negative samples were typed as GII.4 Sydney, GII.13, GI.3, GI.5, GI.2, GII.1, and GII.3 in the reference method.

CONCLUSIONS

The RGN RT-PCR assay had a high sensitivity and specificity for the detection of norovirus in stool specimens from patients with sporadic AGE.

Septins: Regulators of Protein Stability

Septins are small GTPases that play a role in several important cellular processes. In this review, we focus on the roles of septins in protein stabilization. Septins may regulate protein stability by: (1) interacting with proteins involved in degradation pathways, (2) regulating the interaction between transmembrane proteins and cytoskeletal proteins, (3) affecting the mobility of transmembrane proteins in lipid bilayers, and (4) modulating the interaction of proteins with their adaptor or signaling proteins. In this context, we discuss the role of septins in protecting four different proteins from degradation. First we consider botulinum neurotoxin serotype A (BoNT/A) and the contribution of septins to its extraordinarily long intracellular persistence. Next, we discuss the role of septins in stabilizing the receptor tyrosine kinases EGFR and ErbB2. Finally, we consider the contribution of septins in protecting hypoxia-inducible factor 1α (HIF-1α) from degradation.

Increased Susceptibility of Humanized NSG Mice to Panton-Valentine Leukocidin and Staphylococcus aureus Skin Infection

Staphylococcus aureus is a leading cause of skin and soft-tissue infections worldwide. Mice are the most commonly used animals for modeling human staphylococcal infections. However a supra-physiologic S. aureus inoculum is required to establish gross murine skin pathology. Moreover, many staphylococcal factors, including Panton-Valentine leukocidin (PVL) elaborated by community-associated methicillin-resistant S. aureus (CA-MRSA), exhibit selective human tropism and cannot be adequately studied in mice. To overcome these deficiencies, we investigated S. aureus infection in non-obese diabetic (NOD)/severe combined immune deficiency (SCID)/IL2rγ^null (NSG) mice engrafted with human CD34⁺ umbilical cord blood cells. These “humanized” NSG mice require one to two log lower inoculum to induce consistent skin lesions compared with control mice, and exhibit larger cutaneous lesions upon infection with PVL⁺ versus isogenic PVL^-S. aureus. Neutrophils appear important for PVL pathology as adoptive transfer of human neutrophils alone to NSG mice was sufficient to induce dermonecrosis following challenge with PVL⁺S. aureus but not PVL^-S. aureus. PMX53, a human C5aR inhibitor, blocked PVL-induced cellular cytotoxicity in vitro and reduced the size difference of lesions induced by the PVL⁺ and PVL^-S. aureus, but PMX53 also reduced recruitment of neutrophils and exacerbated the infection. Overall, our findings establish humanized mice as an important translational tool for the study of S. aureus infection and provide strong evidence that PVL is a human virulence factor.

S. aureus infection has emerged in the past decade as a major burden to public health and is responsible for a surge in preclinical research. Mice are the most commonly studied animals for modeling of human S. aureus infection. However, it is increasingly evident that available murine models poorly mimic human S. aureus disease. Routinely, a supra-physiologic inoculum is required to establish soft-tissue pathology. Additionally, many S. aureus factors exhibit unique human tropism and cannot be adequately investigated in rodents. Here we investigated S. aureus infection in NSG mice engrafted with human umbilical CD34⁺ cells. We showed that a one to two log lower infectious inoculum of S. aureus establishes consistent skin lesions in humanized NSG mice. This inoculum is comparable to published inocula required to induce infection in humans. In addition, we showed that Panton-Valentine Leucocidin, a human tropic factor secreted by S. aureus, contributes to the development of dermonecrosis in the humanized mice, and its interaction with human neutrophils and human C5a receptor appears to be important for immunopathology. Overall our study suggests that humanized mice are an improved tool for modeling of human S. aureus infection.

Separately or combined, LukG/LukH is functionally unique compared to other staphylococcal bicomponent leukotoxins

Staphylococcus aureus is a major human pathogen that elaborates several exotoxins. Among these are the bicomponent leukotoxins (BCLs), which include γ-hemolysin, Panton-Valentine leukocidin (PVL), and LukDE. The toxin components are classified as either F or S proteins, which are secreted individually and assemble on cell surfaces to form hetero-oligomeric pores resulting in lysis of PMNs and/or erythrocytes. F and S proteins of γ-hemolysin, PVL and LukDE have ∼ 70% sequence homology within the same class and several heterologous combinations of F and S members from these three bicomponent toxin groups are functional. Recently, an additional BCL pair, LukGH (also called LukAB) that has only 30% homology to γ-hemolysin, PVL and LukDE, has been characterized from S. aureus. Our results showed that LukGH was more cytotoxic to human PMNs than PVL. However, LukGH-induced calcium ion influx in PMNs was markedly attenuated and slower than that induced by PVL and other staphylococcal BCLs. In contrast to other heterologous BCL combinations, LukG in combination with heterologous S components, and LukH in combination with heterologous F components did not induce calcium ion entry or cell lysis in human PMNs or rabbit erythrocytes. Like PVL, LukGH induced IL-8 production by PMNs. While individual components LukG and LukH had no cytolytic or calcium influx activity, they each induced high levels of IL-8 transcription and secretion. IL-8 production induced by LukG or LukH was dependent on NF-κB. Therefore, our results indicate LukGH differs functionally from other staphylococcal BCLs.

Outpatient parenteral antimicrobial therapy at large Veterans Administration medical center

OBJECTIVES

To evaluate our outpatient parenteral antimicrobial therapy (OPAT) program to determine its impact on infection management in a facility notable for high patient comorbidity and a large catchment area that includes most of Southern California.

STUDY DESIGN

Retrospective chart review.

METHODS

We reviewed all episodes of patients receiving OPAT from our institution from 2006 through 2009 for patient utilization characteristics and assessment of complications.

RESULTS

A total of 333 patients received 393 courses of OPAT for a mean of 21.1 days. Diabetes mellitus (53.5%), psychiatric disease (39%), and chronic kidney disease (31%) were common; more than half the patients lived more than 20 miles from our medical center. Osteomyelitis (39.7%) and bacteremia (19.3%) accounted for the majority of OPAT indications. Staphylococcus aureus (36.4%) was the most frequent infecting organism, and vancomycin (37.4%) was the most frequently prescribed medication. Complications including hospital readmission, adverse drug reactions, or line-related complications were noted in 96 of 393 (24.4%) episodes, but most were minor, reversible, or not directly related to the OPAT given. Serious line-related complications that required hospital readmission were noted in only 6 (1.5%) episodes. OPAT was completed as planned in 313 (79.6%) episodes; end-stage renal disease was associated with OPAT noncompletion in multivariable analysis (odds ratio = 2.20, P = .047). We estimated that OPAT saved our medical center $4 million per year.

CONCLUSIONS

Despite our patients' high level of comorbidity and our facility's large catchment area, we were able to deliver OPAT successfully and safely with significant cost savings.

In vitro C3 deposition on Cryptococcus capsule occurs via multiple complement activation pathways

Complement can be activated via three pathways: classical, alternative, and lectin. Cryptococcus gattii and Cryptococcus neoformans are closely related fungal pathogens possessing a polysaccharide capsule composed mainly of glucuronoxylomannan (GXM), which serves as a site for complement activation and deposition of complement components. We determined C3 deposition on Cryptococcus spp. by flow cytometry and confocal microscopy after incubation with serum from C57BL/6J mice as well as mice deficient in complement components C4, C3, factor B, and mannose binding lectin (MBL). C. gattii and C. neoformans activate complement in EGTA-treated serum indicating that they can activate the alternative pathway. However, complement activation was seen with factor B(-/-) serum suggesting activation could also take place in the absence of a functional alternative pathway. Furthermore, we uncovered a role for C4 in the alternative pathway activation by Cryptococcus spp. We also identified an unexpected and complex role for MBL in complement activation by Cryptococcus spp. No complement activation occurred in the absence of MBL-A and -C proteins although activation took place when the lectin binding activity of MBL was disrupted by calcium chelation. In addition, alternative pathway activation by C. neoformans required both MBL-A and -C, while either MBL-A or -C was sufficient for alternative pathway activation by C. gattii. Thus, complement activation by Cryptococcus spp. can take place through multiple pathways and complement activation via the alternative pathway requires the presence of C4 and MBL proteins.

Detection and quantification of Panton-Valentine leukocidin in Staphylococcus aureus cultures by ELISA and Western blotting: diethylpyrocarbonate inhibits binding of protein A to IgG

Enzyme-linked immunosorbent assay (ELISA) and Western blotting are common techniques used to detect and quantify proteins in Staphylococcus aureus culture supernatants, such as Panton-Valentine leukocidin (PVL). However, protein A (Spa) secreted by most S. aureus strains may interfere with these assays by binding to the capturing and detecting antibodies. Here, we have shown that the addition of diethylpyrocarbonate (DEPC) inhibits the binding of Spa to rabbit anti-PVL used as the capturing antibody in ELISA. In Western blotting, the presence of DEPC prevented the binding of detecting antibody to Spa. These modified ELISA and Western blot techniques should prove useful for detecting and quantifying proteins in S. aureus culture supernatants.

Human immunoglobulin G2 (IgG2) and IgG4, but not IgG1 or IgG3, protect mice against Cryptococcus neoformans infection

The encapsulated yeast Cryptococcus neoformans is a significant cause of meningitis and death in patients with AIDS. Some murine monoclonal antibodies (MAbs) against the glucuronoxylomannan (GXM) component of the C. neoformans capsular polysaccharide can prolong the lives of infected mice, while others have no effect or can even shorten survival. To date, no one has systematically compared the efficacies of antibodies with the same variable regions and different human constant regions with their unique combination of effector functions in providing protection against murine C. neoformans infection. In the present study, we examined the efficacies of anti-GXM MAbs of the four human immunoglobulin G (IgG) subclasses, which have identical variable regions but differ in their capacities to bind the three types of Fc receptors for IgG (FcgammaR), their abilities to activate complement, and their half-lives. IgG2 and IgG4 anti-GXM prolonged the lives of infected BALB/c mice, IgG3 anti-GXM did not affect animal survival, while mice treated with IgG1 anti-GXM died earlier than mice treated with phosphate-buffered saline or irrelevant isotype-matched MAbs. All MAbs decreased serum GXM in infected animals. Effector pathways traditionally believed to be important in defense against microbes, such as opsonophagocytosis and complement binding, negatively correlated with antibody efficacy. It is generally accepted that human IgG1 has the most favorable combination of effector functions for therapeutic use against infections. Therefore, our findings have significant implications for humanization of the mouse IgG1 currently in clinical trials for cryptococcal meningitis and for the design of antibody therapeutics to treat other infectious diseases as well.

Tumor Necrosis Factor Antagonists: Different Kinetics and/or Mechanisms of Action May Explain Differences in the Risk for Developing Granulomatous Infection

OBJECTIVE

Tumor necrosis factor (TNF) antagonists fall into 2 classes:etanercept (ETA) is a soluble TNF receptor, while infliximab (INF) and adalimumab (ADA) are monoclonal antibodies against TNF. All 3 drugs are effective in treating rheumatoid arthritis. However, these agents have been associated with an increased risk of granulomatous infections, such as tuberculosis and histoplasmosis. Several reports indicate that the incidence of granulomatous infections may potentially be higher in individuals treated with INF than ETA.

METHODS

We conducted a comprehensive literature search (1966 to 2004) to review the role of TNF in normal and disease states, and the mechanisms of action of the TNF inhibitors. Specifically, we searched for possible mechanisms for the apparent increase in granulomatous infections associated with TNF inhibitors and for reasons that there may be differences between them.

RESULTS

Infection may result from a number of differences between ETA and INF or ADA. First, binding avidities are different, with ETA binding in a 1:1 ratio and INF/ADA binding in 2 to 3:1 ratios. Second, the clearances of ADA, ETA, and INF are different, being about 13 times higher for ETA than INF or ADA, thus resulting in higher steady-state drug levels for ADA and INF. Also, the methods of administration are different, intravenously (for INF) versus subcutaneously (for ETA and ADA), which results in lower peak concentrations for ETA and ADA, potentially explaining some of the differences in effects on granuloma formation. Third, INF and ADA have somewhat different mechanisms of action from ETA: INF and ADA are associated with antibody-mediated cell lysis, while ETA is not; INF may induce apoptosis in some tissues (eg, gastrointestinal [GI] mucosa) while ETA does not--although this is controversial and may not be true at steady state in synovium, where both drugs seem to cause apoptosis; ETA binds lymphotoxin-alpha while INF does not (ETA may thus be more efficient at preventing granuloma formation by this mechanism than INF); finally, ADA and INF seem to inhibit IFN-gamma expression (probably indirectly), while ETA does not.

CONCLUSIONS

There are significant differences between the 2 classes of TNF antagonists in terms of both their kinetics and mechanisms of action. These differences may help explain the apparent differences in the incidence of granuloma-dependent infections among them.

Production of human monoclonal antibody in eggs of chimeric chickens

The tubular gland of the chicken oviduct is an attractive system for protein expression as large quantities of proteins are deposited in the egg, the production of eggs is easily scalable and good manufacturing practices for therapeutics from eggs have been established. Here we examined the ability of upstream and downstream DNA sequences of ovalbumin, a protein produced exclusively in very high quantities in chicken egg white, to drive tissue-specific expression of human mAb in chicken eggs. To accommodate these large regulatory regions, we established and transfected lines of chicken embryonic stem (cES) cells and formed chimeras that express mAb from cES cell-derived tubular gland cells. Eggs from high-grade chimeras contained up to 3 mg of mAb that possesses enhanced antibody-dependent cellular cytotoxicity (ADCC), nonantigenic glycosylation, acceptable half-life, excellent antigen recognition and good rates of internalization.

Granulomatous infectious diseases associated with tumor necrosis factor antagonists

The relationship between the use of tumor necrosis factor antagonists and onset of granulomatous infection was examined using data collected through the Adverse Event Reporting System of the US Food and Drug Administration for January 1998-September 2002. Granulomatous infections were reported at rates of approximately 239 per 100,000 patients who received infliximab and approximately 74 per 100,000 patients who received etanercept (P<.001). Tuberculosis was the most frequently reported disease, occurring in approximately 144 and approximately 35 per 100,000 infliximab-treated and etanercept-treated patients, respectively (P<.001). Candidiasis, coccidioidomycosis, histoplasmosis, listeriosis, nocardiosis, and infections due to nontuberculous mycobacteria were reported with significantly greater frequency among infliximab-treated patients. Seventy-two percent of these infection occurred < or =90 days after starting infliximab treatment, and 28% occurred after starting etanercept treatment (P<.001). These data indicate a risk of granulomatous infection that was 3.25-fold greater among patients who received infliximab than among those who received etanercept. The clustering of reports shortly after initiation of treatment with infliximab is consistent with reactivation of latent infection.

Antibodies to keyhole limpet hemocyanin cross-react with an epitope on the polysaccharide capsule of Cryptococcus neoformans and other carbohydrates: implications for vaccine development

Cryptococcus neoformans causes a life-threatening meningoencephalitis in AIDS patients. Mice immunized with a glycoconjugate vaccine composed of the glucuronoxylomannan (GXM) component of the cryptococcal capsular polysaccharide conjugated to tetanus toxoid produce Abs that can be either protective or nonprotective. Because nonprotective Abs block the efficacy of protective Abs, an effective vaccine must focus the Ab response on a protective epitope. Mice immunized with peptide mimetics of GXM conjugated to keyhole limpet hemocyanin (KLH) with glutaraldehyde developed Abs to GXM. However, control peptides P315 and P24 conjugated to KLH also elicited Abs to GXM. GXM-binding Abs from mice immunized with P315-KLH were inhibited by KLH treated with glutaraldehyde (KLH-g), but not by P315. Furthermore, KLH-g inhibited binding of GXM by serum of mice immunized with GXM-TT, indicating that glutaraldehyde treatment of KLH reveals an epitope(s) that cross-reacts with GXM. Vaccination with KLH-g or unmodified KLH elicited Abs to GXM, but did not confer protection against C. neoformans, suggesting the cross-reactive epitope on KLH was not protective. This was supported by the finding that 4H3, a nonprotective mAb, cross-reacted strongly with KLH-g. Sera from mice immunized with either native KLH or KLH-g cross-reacted with several other carbohydrate Ags, many of which have been conjugated to KLH for vaccine development. This study illustrates how mAbs can be used to determine the efficacy of potential vaccines, in addition to describing the complexity of using KLH and glutaraldehyde in the development of vaccines to carbohydrate Ags.

High affinity mimotope of the polysaccharide capsule of Cryptococcus neoformans identified from an evolutionary phage peptide library

Cryptococcus neoformans causes a life-threatening meningoencephalitis in a significant percentage of AIDS patients. Mice immunized with a glycoconjugate vaccine composed of the glucuronoxylomannan (GXM) component of the cryptococcal capsular polysaccharide conjugated to tetanus toxoid (TT) produce Abs that, based on the epitope recognized, can be either protective or nonprotective. Since nonprotective Abs block the efficacy of protective Abs, we are interested in developing a vaccine that would focus the immune response specifically to protective epitopes. Previously, we screened a phage display library with 2H1, a protective anti-GXM mAb, and isolated PA1, a representative peptide that had a K(d) of 295 nM for 2H1. Mice immunized with PA1 conjugated to keyhole limpet hemocyanin developed high anti-peptide (1/13,000), but low anti-GXM (maximum, 1/200) titers. We now report our efforts to improve this vaccine by screening a sublibrary with six random amino acids added to either end of the PA1 motif to identify higher affinity peptides. P206.1, a peptide isolated from this sublibrary, had 80-fold higher affinity for 2H1 (K(d) = 3.7 nM) than PA1. P206.1 bound protective, but not nonprotective, anti-GXM Abs. Mice immunized with P206.1 conjugated to various carriers did not mount an Ab response to GXM despite developing high anti-peptide titers. However, mice primed with GXM-TT and boosted with P206.1-TT developed significant anti-GXM titers (maximum, 1/180,000). This latter immunization scheme focused the immune response on protective epitopes, since only 2-5% of these titers were directed against nonprotective de-O-acetylated GXM epitopes compared with 20-60% in animals primed and boosted with GXM-TT.

Immunoglobulin G monoclonal antibodies to Cryptococcus neoformans protect mice deficient in complement component C3

Passive administration of monoclonal antibodies (MAbs) to the capsular polysaccharide of Cryptococcus neoformans can alter the course of infection in mice. In a murine model of cryptococcal infection, immunoglobulin G1 (IgG1), IgG2a, and IgG2b switch variants of the anti-capsular 3E5 MAb prolong the survival of lethally infected mice, whereas the 3E5 IgG3 MAb does not protect and in some cases enhances infection, shortening the life spans of infected mice. We examined the role of complement component C3 in Ab-mediated protection by determining the efficacy of the four mouse IgG subclasses against C. neoformans in mice genetically deficient in factor C3 as well as mice acutely depleted of C3. Similar to other complement-deficient animal models, C3(-/-) mice and mice depleted of C3 by cobra venom factor were more susceptible to C. neoformans infection than control mice, providing further evidence that complement is important in the host defense against the fungus. In the absence of C3, all IgG isotypes prolonged the lives of mice infected with C. neoformans, indicating that protection by IgG does not require the complement pathways. Furthermore, we observed protection with IgG3 in the complement-deficient mice, suggesting that complement is involved in the lack of protection observed with IgG3 in other mouse models.

Both Th1 and Th2 cytokines affect the ability of monoclonal antibodies to protect mice against Cryptococcus neoformans

Variable-region-identical mouse immunoglobulin G1 (IgG1), IgG2b, and IgG2a monoclonal antibodies to the capsular polysaccharide of Cryptococcus neoformans prolong the lives of mice infected with this fungus, while IgG3 is either not protective or enhances infection. CD4+ T cells are required for IgG1-mediated protection, and CD8+ T cells are required for IgG3-mediated enhancement. Gamma interferon is required for both effects. These findings revealed that T cells and cytokines play a role in the modulation of cryptococcal infection by antibodies and suggested that it was important to more fully define the cytokine requirements of each of the antibody isotypes. We therefore investigated the efficacy of passively administered variable-region-identical IgG1, IgG2a, IgG2b, and IgG3 monoclonal antibodies against intravenous infection with C. neoformans in mice genetically deficient in interleukin-12 (IL-12), IL-6, IL-4, or IL-10, as well as in the parental C57BL/6J strain. The relative inherent susceptibilities of these mouse strains to C. neoformans were as follows: IL-12(-/-) > IL-6(-/-) > C57BL/6J approximately IL-4(-/-) >> IL-10(-/-). This is consistent with the notion that a Th1 response is necessary for natural immunity against cryptococcal infection. However, none of the IgG isotypes prolonged survival in IL-12(-/-), IL-6(-/-), or IL-4(-/-) mice, and all isotypes significantly enhanced infection in IL-10(-/-) mice. These results indicate that passive antibody-mediated protection against C. neoformans requires both Th1- and Th2-associated cytokines and reveal the complexity of the mechanisms through which antibodies modulate infection with this organism.

Anti-HIV-1 activity of 3'-azido-3'-deoxythymidine (AZT) in primary mononuclear phagocytes

Conflicting data have been reported on ability of 3'-azido-3'-deoxythymidine (AZT) to protect mononuclear phagocytes from HIV-1 infection. We compared the antiviral potency of AZT in three types of primary human mononuclear phagocytes: peripheral blood monocytes, monocyte-derived macrophages (in vitro differentiated) and alveolar macrophages (in vivo differentiated). To establish highly-productive virus infection, purified cells (greater than 99%) from healthy donors were challenged with the macrophage-tropic HTLV-IIIBa-L strain at input multiplicities ranging from 0.05 to 20 TCID50 per cell. AZT (0.1 nM-10 microM) was added immediately after infection and either continued for the duration of the experiment or stopped 1-7 days after infection. The kinetics of HIV-1Ba-L replication were assessed by measuring p24 antigen production on days 4-28 post-infection. Continuous treatment with AZT reproducibly inhibited viral replication in a concentration-dependent manner in all three cell types. The IC90 of AZT was 0.04 microM in blood monocytes, 0.009 microM in monocyte-derived macrophages, and 0.0001 microM in alveolar macrophages (mean of 3-4 donors for each cell type). AZT was not cytotoxic at less than 10 microM as assessed by cell viability, cell protein, and interferon-gamma-activated H2O2-release. In experiments in which AZT treatment was stopped after infection, viral replication resumed after a lag of 7-14 days and increased exponentially toward control levels. This occurred despite initial inhibition of virus production to below the limit of p24 detection (approximately 50 pg/ml). These results indicate that AZT is a potent inhibitor of HIV-1 replication in primary mononuclear phagocytes regardless of the stage of cell differentiation, and that AZT is most active in tissue (alveolar) macrophages. AZT does not irreversibly block infection of mononuclear phagocytes, however, as viral replication resumes after removal of AZT.