Urban green waste bulking agent is the major source of antimicrobial resistance genes persisted in home compost, not animal manure

Urban green waste and food waste are often used as bulking agents to prepare home compost in combination with animal manure in urban horticulture and community gardening. Although it is known that antimicrobial resistance genes (ARGs) persist in home compost, their origins have not been determined. In addition, the factors contributing to ARGs persistence remain unclear. In this study, we aim to (i) characterize the changes in the microbiome and antimicrobial resistome during the composting process of home compost using metagenomics shotgun sequencing, (ii) identify the source of the ARGs persisted in home compost using SourceTracker, and (iii) elucidate the collective effect of compost microbiome and environmental factors, including the physicochemical properties and antibiotics concentration of home compost, in contributing to ARG persistence using Procrustes analysis, co-occurrence network analysis, variation partitioning analysis, and structural equation modeling. SourceTracker analysis indicated that urban green waste bulking agent was the major source of the persisting ARGs in home compost instead of animal manure. Procrustes analysis and co-occurrence network analysis revealed a strong association between microbiome and antimicrobial resistome. Variation partitioning analysis and structural equation modeling suggested that physicochemical properties shaped the antimicrobial resistome directly and indirectly by influencing the microbiome. Our results indicated that the persistence of ARGs in home compost might be due to the succession of microbial species from the urban green waste bulking agent, and the physicochemical properties might have defined the compost environment to shape the microbiome in the compost, thus, in turn, the persisting antimicrobial resistome.

Pharmacokinetics of Oral Nirmatrelvir/Ritonavir, a Protease Inhibitor for Treatment of COVID-19, in Subjects With Renal Impairment

Nirmatrelvir coadministered with ritonavir is highly efficacious in reducing the risk of coronavirus disease 2019 (COVID‐19) adverse outcomes among patients at increased risk of progression to severe disease, including patients with chronic kidney disease. Because nirmatrelvir is eliminated by the kidneys when given with ritonavir, this phase I study evaluated the effects of renal impairment on pharmacokinetics, safety, and tolerability of nirmatrelvir/ritonavir. Participants with normal renal function (n = 10) or mild, moderate, or severe renal impairment (n = 8 each) were administered a single 100‐mg nirmatrelvir dose with 100 mg ritonavir given 12 hours before, together with and 12 and 24 hours after the nirmatrelvir dose. Systemic nirmatrelvir exposure increased with increasing renal impairment, with mild, moderate, and severe renal impairment groups having respective adjusted geometric mean ratio areas under the plasma concentration‐time profile from time 0 extrapolated to infinite time of 124%, 187%, and 304% vs. the normal renal function group. Corresponding ratios for maximum plasma concentration were 130%, 138%, and 148%. Apparent clearance was positively correlated with estimated glomerular filtration rate, and geometric mean renal clearance values were particularly lower for the moderate (47% decrease) and severe (80% decrease) renal impairment groups vs. the normal renal function group. Nirmatrelvir/ritonavir exhibited an acceptable safety profile; treatment‐related adverse events were mild in severity, and there were no significant findings regarding laboratory measurements, vital signs, or electrocardiogram assessments. These findings led to a dose reduction recommendation for nirmatrelvir/ritonavir in patients with moderate renal impairment (150/100 mg nirmatrelvir/ritonavir instead of 300/100 mg twice daily for 5 days). NCT04909853.

Population Pharmacokinetic Modeling and Probability of Pharmacodynamic Target Attainment for Ceftazidime-Avibactam in Pediatric Patients Aged 3 Months and Older

Increasing prevalence of infections caused by antimicrobial-resistant gram-negative bacteria represents a global health crisis, and while several novel therapies that target various aspects of antimicrobial resistance have been introduced in recent years, few are currently approved for children. Ceftazidime-avibactam is a novel β-lactam β-lactamase inhibitor combination approved for adults and children 3 months and older with complicated intra-abdominal infection, and complicated urinary tract infection or hospital-acquired ventilator-associated pneumonia (adults only in the United States) caused by susceptible gram-negative bacteria. Extensive population pharmacokinetic (PK) data sets for ceftazidime and avibactam obtained during the adult clinical development program were used to iteratively select, modify, and validate the approved adult dosage regimen (2,000-500 mg by 2-hour intravenous (IV) infusion every 8 hours (q8h), with adjustments for renal function). Following the completion of one phase I (NCT01893346) and two phase II ceftazidime-avibactam studies (NCT02475733 and NCT02497781) in children, adult PK data sets were updated with pediatric PK data. This paper describes the development of updated combined adult and pediatric population PK models and their application in characterizing the population PK of ceftazidime and avibactam in children, and in dose selection for further pediatric evaluation. The updated models supported the approval of ceftazidime-avibactam pediatric dosage regimens (all by 2-hour IV infusion) of 50-12.5 mg/kg (maximum 2,000-500 mg) q8h for those ≥6 months to 18 years old, and 40-10 mg/kg q8h for those ≥3 to 6 months old with creatinine clearance > 50 mL/min/1.73 m2 .

The use of extrapolation based on modeling and simulation to support high-dose regimens of ceftaroline fosamil in pediatric patients with complicated skin and soft-tissue infections

A model-informed drug development approach was used to select ceftaroline fosamil high-dose regimens for pediatric patients with complicated skin and soft-tissue infections caused by Staphylococcus aureus with a ceftaroline minimum inhibitory concentration (MIC) of 2 or 4 mg/L. Steady-state ceftaroline concentrations were simulated using a population pharmacokinetics (PK) model for ceftaroline fosamil and ceftaroline including data from 304 pediatric subjects and 944 adults. Probability of target attainment (PTA) for various simulated pediatric high-dose regimens and renal function categories were calculated based on patients achieving 35% fT>MIC (S. aureus PK/pharmacodynamic target for 2-log10 bacterial killing). For extrapolation of efficacy, simulated exposures and PTA were compared to adults with normal renal function receiving high-dose ceftaroline fosamil (600 mg 2-h infusions every 8 h). For safety, predicted ceftaroline exposures were compared with observed pediatric and adult data. Predicted ceftaroline exposures for the approved pediatric high-dose regimens (12, 10, or 8 mg/kg by 2-h infusions every 8 h for patients aged >2 to <18 years with normal/mild, moderate, or severe renal impairment, respectively; 10 mg/kg by 2-h infusions every 8 h for patients aged ≥2 months to <2 years with normal renal function/mild impairment) were well matched to adults with normal renal function. Median predicted maximum concentration at steady state (Cmax,ss ) and area under the plasma concentration-time curve over 24 h at steady state pediatric to adult ratios were 0.907-1.33 and 0.940-1.41, respectively. PTAs (>99% and ≥81% for MICs of 2 and 4 mg/L, respectively) matched or exceeded the adult predictions. Simulated Cmax,ss values were below the maximum observed data in other indications, including a high-dose pediatric pneumonia trial, which reported no adverse events related to high exposure.

Pharmacokinetic and Pharmacodynamic Target Attainment in Adult and Pediatric Patients Following Administration of Ceftaroline Fosamil as a 5-Minute Infusion

The key pharmacokinetic/pharmacodynamic (PK/PD) efficacy index for β-lactam antibiotics is the percentage of time that free drug concentrations exceed the minimum inhibitory concentration (MIC) of bacteria during each dosing interval (fT>MIC). Ceftaroline fosamil, the prodrug of the β-lactam ceftaroline, was initially approved for administration as 60-minute intravenous (IV) infusions. Population PK analyses comparing exposure and PK/PD target attainment for 5-minute and 60-minute IV infusions, described here, have supported ceftaroline fosamil labeling updates to include variable infusion durations of 5 to 60 minutes in adults and children aged ≥2 months. A 2-compartment disposition PK model for ceftaroline fosamil and ceftaroline was used to predict steady-state ceftaroline exposures (maximum plasma concentrations [Cmax,ss ] and area under the plasma concentration-time curve over 24 hours [AUCss,0-24 ]) and probability of target attainment in simulated adult and pediatric patients with various degrees of renal function receiving standard doses of ceftaroline fosamil as 5-minute or 60-minute IV infusions. Across age groups and renal function categories, median ceftaroline AUCss,0-24 values were similar for 5-minute and 60-minute infusions, whereas Cmax,ss was up to 42% higher for 5-minute infusions. Both infusion durations achieved >99% probability of target attainment based on PK/PD targets for Staphylococcus aureus (35% fT>MIC) and Streptococcus pneumoniae (44% fT>MIC) at European Committee on Antimicrobial Susceptibility Testing/Clinical and Laboratory Standards Institute MIC breakpoints (1 mg/L and 0.25/0.5 mg/L, respectively). These findings support administration of standard ceftaroline fosamil doses over 5 to 60 minutes for adults and children aged ≥2 months, providing added flexibility to clinicians and patients.

Saikosaponin-d, a novel SERCA inhibitor, induces autophagic cell death in apoptosis-defective cells

Autophagy is an important cellular process that controls cells in a normal homeostatic state by recycling nutrients to maintain cellular energy levels for cell survival via the turnover of proteins and damaged organelles. However, persistent activation of autophagy can lead to excessive depletion of cellular organelles and essential proteins, leading to caspase-independent autophagic cell death. As such, inducing cell death through this autophagic mechanism could be an alternative approach to the treatment of cancers. Recently, we have identified a novel autophagic inducer, saikosaponin-d (Ssd), from a medicinal plant that induces autophagy in various types of cancer cells through the formation of autophagosomes as measured by GFP-LC3 puncta formation. By computational virtual docking analysis, biochemical assays and advanced live-cell imaging techniques, Ssd was shown to increase cytosolic calcium level via direct inhibition of sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase pump, leading to autophagy induction through the activation of the Ca(2+)/calmodulin-dependent kinase kinase-AMP-activated protein kinase-mammalian target of rapamycin pathway. In addition, Ssd treatment causes the disruption of calcium homeostasis, which induces endoplasmic reticulum stress as well as the unfolded protein responses pathway. Ssd also proved to be a potent cytotoxic agent in apoptosis-defective or apoptosis-resistant mouse embryonic fibroblast cells, which either lack caspases 3, 7 or 8 or had the Bax-Bak double knockout. These results provide a detailed understanding of the mechanism of action of Ssd, as a novel autophagic inducer, which has the potential of being developed into an anti-cancer agent for targeting apoptosis-resistant cancer cells.

Comprehensive characterization of indoor airborne bacterial profile

This is the first detailed characterization of the airborne bacterial profiles in indoor environments. Two restaurants were selected for this study. Fifteen genera of bacteria were isolated from each restaurant and identified by three different bacterial identification systems including MIDI, Biolog and Riboprinter. The dominant bacteria of both restaurants were Gram-positive bacteria in which Micrococcus and Bacillus species were the most abundant. Most bacteria identified were representative species of skin and respiratory tract of human, and soil. Although the bacterial levels in these two restaurants were below the limit of the Hong Kong Indoor Air Quality Objective (HKIAQO) Level 1 standard (i.e., < 500 cfu/m3), the majority of these bacteria were opportunistic pathogens. These results suggested that the identity of airborne bacteria should also be included in the IAQ to ensure there is a safety guideline for the public.

Microbial synthesis and characterization of physiochemical properties of polyhydroxyalkanoates (PHAs) produced by bacteria isolated from activated sludge obtained from the municipal wastewater works in Hong Kong

The first objective of this study was the measurement of physical properties of P(3HB-co-3HV) copolymers with different (hydroxybutyrate) HB to (hydroxyvalerate) HV ratios produced by Bacillus cereus (TRY2) isolated from activated sludge. The 3HV PHBV copolymers were 0.05, 22.6, 39.2, 54.1, and 69.1 mol%, respectively. The second objective was to study possible waste water treatment and production of PHAs at the same time by B. cereus (TRY2) and Pseudomonas spp. (TOB17) (both were isolated from activated sludge), recombinant Bacillus DH5alpha, and a combination of the above three bacteria. The results were satisfactory; the maximum COD and TOC of the sewage sludge reduced were 53.5% and 67.5%, respectively.

A comparison of the prevalence of domestic violence between patients seeking termination of pregnancy and other general gynecology patients

OBJECTIVES

(1) To determine and compare the prevalence of domestic violence among abortion-seeking patients with other general gynecology patients; (2) to see if a follow-up interview 6 weeks after abortion can improve the abuse disclosure rate; (3) to see if the abused victims accept direct referral to their gynecologists/social workers for help.

METHODS

This is a prospective questionnaire survey in a university teaching hospital on patients seeking abortion and an approximately equal number of other general gynecology patients. Participants were interviewed by a designated project nurse in a private setting, using a structured questionnaire (Modified Abuse Assessment Screen Questionnaire) to assess the past and recent history of abuse. The interview was repeated 6 weeks after the abortion for the abortion-seeking group.

RESULTS

Five hundred and one participants were interviewed, including 245 seeking abortion (TOP group) and 256 other general gynecology patients (non-TOP group). The lifetime prevalence of abuse in the TOP group (27.3%) was significantly higher than the non-TOP group (8.2%) (P<0.001). Repeating the interview 6 weeks after the abortion did not increase the disclosure rate. Most abused victims were unwilling to disclose their information of abuse to their gynecologists or social workers at the time of interview.

CONCLUSIONS

Domestic violence is a significant problem among the gynecology patients, particularly those seeking abortion. A single interview prior to abortion is adequately effective for screening. However, the most effective and acceptable way of helping these victims needs to be explored further.

Drug treatment effects on disease progression

Degenerative diseases are characterized by a worsening of disease status over time. The rate of deterioration is determined by the natural rate of progression of the disease and by the effect of drug treatments. A goal of drug treatment is to slow disease progression. Drug treatments can be categorized as symptomatic or protective. Symptomatic treatments do not affect the rate of disease progression whereas protective treatments have the ability to slow disease progression down. Many current methods for describing disease progression have two common drawbacks: a linear relationship between time and disease status is assumed, and within- and between-subject variability is ignored. Disease progress models combined with pharmacokinetic pharmacodynamic models and hierarchical random effects statistical models provide insights into understanding the time course and management of degenerative disease.

Intubation through intubating laryngeal mask with and without a lightwand: a randomized comparison

The combined use of a lightwand and the intubating laryngeal mask airway (ILMA) was compared with the use of the ILMA alone to determine whether the combination was a more efficient method of endotracheal intubation. One hundred healthy patients were randomly assigned to two groups. After induction of anaesthesia, Group A patients were intubated blindly through the ILMA while in Group B, intubation was guided by a lightwand. A sequence of standard manoeuvres was followed it attempts at intubation failed. The number of manoeuvres used, the time taken for successful intubation and complications associated with intubation were recorded. Intubations were successful in all patients, but the mean endotracheal intubation time was longer in Group A than in Group B (38.3 +/- 10.4 s versus 26.4 +/- 9.1 s, P < 0.001). The number of patients who needed one or more manoeuvres was significantly higher in Group A than in Group B (76% versus 42%, P = 0.001). We conclude that the lightwand is a useful adjunct in endotracheal intubation through an ILMA.

Gestational diabetes mellitus in the last trimester - a feature of maternal iron excess?

AIM

To determine whether non-anaemic women with gestational diabetes mellitus (GDM) diagnosed in third trimester pregnancy have evidence of increased iron stores compared with matched non-diabetic controls.

METHODS

In a prospective study, women who had antenatal booking before 20 weeks' gestation and without anaemia or diabetes mellitus were recruited at the time of the oral glucose tolerance test (OGTT) at 28-31 weeks' gestation for the study of serum ferritin, iron and transferrin concentrations. The results were blinded to the managing obstetricians. After delivery, the records were reviewed. The cases diagnosed as GDM were compared with a control group (two controls for each index case matched for parity) selected at random from the at-risk but nondiabetic cases.

RESULTS

GDM was diagnosed in 97 of the 401 women recruited. Compared with the 194 controls, there was no difference in the weight, body mass index, booking and third trimester haemoglobin, or third trimester red cell indices, but concentrations of serum ferritin, iron, transferrin saturation, and the post-natal haemoglobin were significantly higher. On multiple regression analysis, maternal BMI and the log-transformed ferritin concentration remained significant determinants of the OGTT 2-h glucose value.

CONCLUSION

The results suggest an association between increased iron stores and glucose intolerance at the third trimester in non-anaemic women. The role of iron excess in the pathogenesis of GDM needs to be examined.

Paracetamol plasma and cerebrospinal fluid pharmacokinetics in children

AIMS

Paracetamol has a central action for both antipyresis and analgesia. Maximum temperature decrease and peak analgesia are reported at 1-2 h after peak plasma paracetamol concentration. We wished to determine the relationship between plasma and cerebrospinal fluid (CSF) pharmacokinetics in children.

METHODS

Concentration-time profiles in plasma and CSF after nasogastric paracetamol 40 mg kg(-1) were measured in nine children who had indwelling ventricular drains. Estimation of population pharmacokinetic parameters was made using both a standard two-stage population approach (MKMODEL) and a nonlinear mixed effect model (NONMEM). Results were standardized to a 70 kg person using an allometric power model.

RESULTS

Both approaches gave similar estimates. NONMEM parameter estimates were clearance 10.21 h(-1) (CV 47%), volume of distribution 67.11 (CV 58%) and absorption rate constant 0.77 h(-1) (CV 49%). Cerebrospinal fluid concentrations lagged behind those of plasma. The equilibration half time was 0.72 h (CV 117%). The CSF/plasma partition coefficient was 1.18 (CV 8%).

CONCLUSIONS

Higher concentrations in the CSF probably reflect the lower free water volume of plasma. The CSF equilibration half time suggests that CSF kinetics approximate more closely to the effect compartment than plasma, but further time is required for paracetamol to exert its effects. Effect site concentrations equilibrate slowly with plasma. Paracetamol should be given 1-2 h before anticipated pain or fever in children.

Assay of Salmonella in enrichment cultures of foods, feeds and environmental samples by the polymyxin-cloth enzyme immunoassay

A variety of foods, animal feeds and environmental samples were analyzed for the presence of Salmonella using the polymyxin-cloth enzyme immunoassay (p-CEIA) system. Salmonella lipopolysaccharide (LPS) antigens were captured from test samples on polymyxin-coated polyester cloth, followed by immunoenzymatic detection of bound antigens using a monoclonal antibody recognizing an LPS common core oligosaccharide. Dairy and egg products, animal feeds and environmental samples from food processing plants were pre-enriched for 24 h, followed by selective enrichment for a further 24 h in either tetrathionate brilliant green (TBG), selenite cystine (SC) or brain-heart infusion broth containing 0.5% yeast extract, 0.5% cholate and 0.3% selenite (BYCS). The samples were assayed by the p-CEIA after each stage of enrichment. After selective enrichment, the p-CEIA gave results which were in complete agreement with the standard culture technique in the analysis of all foods examined. On the other hand, a combination of selective enrichment and the p-CEIA out-performed the Modified Semi-Solid Rappaport Vassiliadis (MSRV) method in screening pre-enrichment cultures of feeds and environmental samples. Use of the new selective medium BYCS prior to performing the p-CEIA gave the highest recovery of Salmonella from feeds and environmental samples.

Value of standardized gestational age charts for fetuses of first-generation Oriental immigrants to Canada

OBJECTIVE

to ascertain if standard gestational age charts can be used to accurately predict the gestational age of fetuses of first-generation Oriental immigrants to Canada.

PATIENTS AND METHODS

Over a 3-year period, all patients presenting for obstetric ultrasound examinations were invited to participate in the study. The authors recorded biparietal diameter, head circumference, femur length and abdominal circumference for 139 fetuses in the second and third trimesters. A study performed in the first trimester was used as the baseline for gestational age. The data for 126 of the fetuses were complete, and these data were used for the analysis.

RESULTS

For 77 of the fetuses, both parents were Oriental, and for 49, one or both parents were not Oriental; the latter constituted the control group. A total of 1008 individual measurements were obtained, and of these, all but 14 fell within two standard deviations of the norm, according to standard gestational age charts. Of the abnormal measurements, seven were obtained from five fetuses with Oriental parents, and seven were obtained from six fetuses in the control group. The difference between the two groups in the proportion of measurements falling either above or below two standard deviations from the norm was not statistically significant (Fisher's exact test, p = 0.41694).

CONCLUSION

The authors conclude that standard gestational age charts can be used to determine gestational age in first-generation Oriental immigrants to Canada.

Use of limiting-dilution type long-term marrow cultures in frequency analysis of marrow-repopulating and spleen colony-forming hematopoietic stem cells in the mouse

We have developed an in vitro clonal assay of murine hematopoietic precursor cells that form spleen colonies (CFU-S day 12) or produce in vitro clonable progenitors in the marrow (MRA cells) of lethally irradiated mice. The assay is essentially a long-term bone marrow culture in microtiter wells containing marrow-derived stromal "feeders" depleted for hematopoietic activity by irradiation. To test the validity of the assay as a quantitative in vitro stem cell assay, a series of unsorted and physically sorted bone marrow cells were simultaneously assayed in vivo and overlaid on the feeders in a range of concentrations, while frequencies of cells forming hematopoietic clones (cobblestone area forming cells, CAFC) were calculated by means of Poisson statistics. Linear regression analysis of the data showed high correlations between the frequency of CFU-S day 12 and CAFC day 10, and between MRA cells and CAFC day 28. A majority of MRA activity and CAFC day 28 was separable from CFU-S day 12 and CAFC day 10. This correlation study validates the CAFC system as a clonal assay facilitation both the quantitative assessment of a series of subsets in the hematopoietic stem cell hierarchy and the study of single long-term repopulating cells in vitro.

Physical aspects of wet granulations II: factors involved in prolonged and excessive mixing

Evidence is presented that excessive blending in a wet granulation process shifts the packing arrangement of the wet granule, causing it to become dense and nonporous. With prolonged kneading, a large amount of the water-soluble excipients dissolves in the granulating fluid, and these two factors make the drying slower. This result, coupled with the previous finding that a certain time is required to attain an equilibrium size granule, explains why there exists an optimum kneading time for a wet granulation from a mechanical performance point of view.

Interference with antibody-feedback by irradiation, thymus cells, the allogeneic effect, and serum factors

Low doses of irradiation, increased number of T cells, increased T cell activity as in the allogeneic effect, or early activation of an in vitro cell-mediated immune response all confer resistance to antibody feedback. In some of these systems, the resistance is directed against immunosuppression by intact IgG antibody whereas suppression by F(ab')2 antibody is either not changed or increased. These forms of interference with immunosuppression by antibody, are explained in terms of an Fc-dependent model for antibody-feedback regulation.

Effects of a progestogen and a sequential type oral contraceptive on plasma vitamin A, vitamin E, cholesterol and triglycerides

Fasting blood samples were taken from 13 college students who had never been on oral contraceptives in two menstrual cycles. During the first cycle, the control cycle, each girl donated three blood samples; the first sample was given between days 1 and 5, the second sample between days 13 and 17, and the third sample between days 22 and 26 of the menstrual cycle. In the second menstrual cycle, the experimental cycle, nine girls were given Micronor, a progestogen type oral contraceptive and four girls were given Ortho-Novum SQ, a sequential type oral contraceptive. Four blood samples were obtained from each of the subjects: the first three samples were obtained in the three periods corresponding to those in the control cycle, and the fourth was taken 2 days after the subjects had stopped taking the oral contraceptive. Results showed that estrogen significantly raised plasma vitamin A and triglycerides. The progestogen, at low concentration, had little or no effect on these two lipid materials. At a higher concentration the progestogen enhanced the effect of the estrogen on plasma vitamin A and triglycerides. These effects were extended to at least 2 days after subjects had ceased taking the oral contraceptive. Plasma vitamin E levels in the subjects given Ortho-Novum SQ, were consistently but not statistically higher in the experimental cycle than in the control cycle. The correlation coefficient between vitamin E and triglycerides was statistically significant while those between the vitamin A and vitamin E and between vitamin A and triglycerides were not. Cholesterol was not affected by either Micronor or Ortho-Novum SQ.

Regulation of the immune response. VI. Inability of F(ab') 2 antibody to terminate established immune responses and its ability to interfere with IgG antibody-mediated immunosuppression

Intact IgG antibody can terminate established immune responses, whereas F(ab′)₂ antibody cannot do so. The difference between the two antibodies appears to be qualitative. F(ab′)₂ antibody, but not pepsin-digested normal serum, can interfere with the suppression and termination of immune responses induced by intact IgG antibody. These results are discussed in terms of the tripartite inactivation model.

Regulation of the immune response. II. Further studies on differences in ability of F(ab')2 and 7S antibodies to inhibit an antibody response

The ability of F(ab′)₂ antibody preparations to suppress an immune response is much less than that of intact 7S antibody. The activity possessed by F(ab′)₂ preparations withstood repurification procedures, hence contamination with intact 7S antibody is unlikely. Daily or thrice daily injections of antibody did not make equal the suppressive activities of F(ab′)₂ and intact antibody, indicating that rapid excretion of F(ab′)₂ antibody is not the sole factor involved in the difference in immunosuppressive potency between intact 7S and F(ab′)₂ antibody. Some possibilities for distinct differences in the mechanism of the immuno-suppressive action of F(ab′)₂ and 7S antibodies are raised and discussed.