Global phylogeography and ancient evolution of the widespread human gut virus crAssphage

Microbiomes are vast communities of microorganisms and viruses that populate all natural ecosystems. Viruses have been considered to be the most variable component of microbiomes, as supported by virome surveys and examples of high genomic mosaicism. However, recent evidence suggests that the human gut virome is remarkably stable compared with that of other environments. Here, we investigate the origin, evolution and epidemiology of crAssphage, a widespread human gut virus. Through a global collaboration, we obtained DNA sequences of crAssphage from more than one-third of the world's countries and showed that the phylogeography of crAssphage is locally clustered within countries, cities and individuals. We also found fully colinear crAssphage-like genomes in both Old-World and New-World primates, suggesting that the association of crAssphage with primates may be millions of years old. Finally, by exploiting a large cohort of more than 1,000 individuals, we tested whether crAssphage is associated with bacterial taxonomic groups of the gut microbiome, diverse human health parameters and a wide range of dietary factors. We identified strong correlations with different clades of bacteria that are related to Bacteroidetes and weak associations with several diet categories, but no significant association with health or disease. We conclude that crAssphage is a benign cosmopolitan virus that may have coevolved with the human lineage and is an integral part of the normal human gut virome.

Global phylogeography and ancient evolution of the widespread human gut virus crAssphage

Microbiomes are vast communities of microorganisms and viruses that populate all natural ecosystems. Viruses have been considered to be the most variable component of microbiomes, as supported by virome surveys and examples of high genomic mosaicism. However, recent evidence suggests that the human gut virome is remarkably stable compared with that of other environments. Here, we investigate the origin, evolution and epidemiology of crAssphage, a widespread human gut virus. Through a global collaboration, we obtained DNA sequences of crAssphage from more than one-third of the world's countries and showed that the phylogeography of crAssphage is locally clustered within countries, cities and individuals. We also found fully colinear crAssphage-like genomes in both Old-World and New-World primates, suggesting that the association of crAssphage with primates may be millions of years old. Finally, by exploiting a large cohort of more than 1,000 individuals, we tested whether crAssphage is associated with bacterial taxonomic groups of the gut microbiome, diverse human health parameters and a wide range of dietary factors. We identified strong correlations with different clades of bacteria that are related to Bacteroidetes and weak associations with several diet categories, but no significant association with health or disease. We conclude that crAssphage is a benign cosmopolitan virus that may have coevolved with the human lineage and is an integral part of the normal human gut virome.

Gut microbiome composition of wild western lowland gorillas is associated with individual age and sex factors

OBJECTIVES

Environmental and ecological factors, such as geographic range, anthropogenic pressure, group identity, and feeding behavior are known to influence the gastrointestinal microbiomes of great apes. However, the influence of individual host traits such as age and sex, given specific dietary and social constraints, has been less studied. The objective of this investigation was to determine the associations between an individual's age and sex on the diversity and composition of the gut microbiome in wild western lowland gorillas.

MATERIALS AND METHODS

Publicly available 16S rRNA data generated from fecal samples of different groups of Gorilla gorilla gorilla in the Central African Republic were downloaded and bioinformatically processed. The groups analyzed included habituated, partially habituated and unhabituated gorillas, sampled during low fruit (dry, n = 28) and high fruit (wet, n = 82) seasons. Microbial community analyses (alpha and beta diversity and analyses of discriminant taxa), in tandem with network-wide approaches, were used to (a) mine for specific age and sex based differences in gut bacterial community composition and to (b) asses for gut community modularity and bacterial taxa with potential functional roles, in the context of seasonal food variation, and social group affiliation.

RESULTS

Both age and sex significantly influenced gut microbiome diversity and composition in wild western lowland gorillas. However, the largest differences were observed between infants and adults in habituated groups and between adults and immature gorillas within all groups, and across dry and wet seasons. Specifically, although adults always showed greater bacterial richness than infants and immature gorillas, network-wide analyses showed higher microbial community complexity and modularity in the infant gorilla gut. Sex-based microbiome differences were not evident among adults, being only detected among immature gorillas.

CONCLUSIONS

The results presented point to a dynamic gut microbiome in Gorilla spp., associated with ontogeny and individual development. Of note, the gut microbiomes of breastfeeding infants seemed to reflect early exposure to complex, herbaceous vegetation. Whether increased compositional complexity of the infant gorilla gut microbiome is an adaptive response to an energy-limited diet and an underdeveloped gut needs to be further tested. Overall, age and sex based gut microbiome differences, as shown here, maybe mainly attributed to access to specific feeding sources, and social interactions between individuals within groups.

The gut microbiome of nonhuman primates: Lessons in ecology and evolution

The mammalian gastrointestinal (GI) tract is home to trillions of bacteria that play a substantial role in host metabolism and immunity. While progress has been made in understanding the role that microbial communities play in human health and disease, much less attention has been given to host-associated microbiomes in nonhuman primates (NHPs). Here we review past and current research exploring the gut microbiome of NHPs. First, we summarize methods for characterization of the NHP gut microbiome. Then we discuss variation in gut microbiome composition and function across different NHP taxa. Finally, we highlight how studying the gut microbiome offers new insights into primate nutrition, physiology, and immune system function, as well as enhances our understanding of primate ecology and evolution. Microbiome approaches are useful tools for studying relevant issues in primate ecology. Further study of the gut microbiome of NHPs will offer new insight into primate ecology and evolution as well as human health.

Social behaviour and gut microbiota in red-bellied lemurs (Eulemur rubriventer): In search of the role of immunity in the evolution of sociality

Vertebrate gut microbiota form a key component of immunity and a dynamic link between an individual and the ecosystem. Microbiota might play a role in social systems as well, because microbes are transmitted during social contact and can affect host behaviour. Combining methods from behavioural and molecular research, we describe the relationship between social dynamics and gut microbiota of a group-living cooperative species of primate, the red-bellied lemur (Eulemur rubriventer). Specifically, we ask whether patterns of social contact (group membership, group size, position in social network, individual sociality) are associated with patterns of gut microbial composition (diversity and similarity) between individuals and across time. Red-bellied lemurs were found to have gut microbiota with slight temporal fluctuations and strong social group-specific composition. Contrary to expectations, individual sociality was negatively associated with gut microbial diversity. However, position within the social network predicted gut microbial composition. These results emphasize the role of the social environment in determining the microbiota of adult animals. Since social transmission of gut microbiota has the potential to enhance immunity, microbiota might have played an escalating role in the evolution of sociality.

Patterns in Gut Microbiota Similarity Associated with Degree of Sociality among Sex Classes of a Neotropical Primate

Studies of human and domestic animal models indicate that related individuals and those that spend the most time in physical contact typically have more similar gut microbial communities. However, few studies have examined these factors in wild mammals where complex social dynamics and a variety of interacting environmental factors may impact the patterns observed in controlled systems. Here, we explore the effect of host kinship and time spent in social contact on the gut microbiota of wild, black howler monkeys (Alouatta pigra). Our results indicate that closely related individuals had less similar gut microbial communities than non-related individuals. However, the effect was small. In contrast, as previously reported in baboons and chimpanzees, individuals that spent more time in contact (0 m) and close proximity (0-1 m) had more similar gut microbial communities. This pattern was driven by adult female-adult female dyads, which generally spend more time in social contact than adult male-adult male dyads or adult male-adult female dyads. Relative abundances of individual microbial genera such as Bacteroides, Clostridium, and Streptococcus were also more similar in individuals that spent more time in contact or close proximity. Overall, our data suggest that even in arboreal primates that live in small social groups and spend a relatively low proportion of their time in physical contact, social interactions are associated with variation in gut microbiota composition. Additionally, these results demonstrate that within a given host species, subgroups of individuals may interact with the gut microbiota differently.

Fusarium species and fumonisins associated with maize kernels produced in Rio Grande do Sul State for the 2008/09 and 2009/10 growing seasons

Ear rots caused by Fusarium spp. are among the main fungal diseases that contribute to poor quality and the contamination of maize grains with mycotoxins. This study aimed to determine the visual incidence of fungal-damaged kernels (FDKs), the incidence of two main Gibberella (a teleomorph of Fusarium) complexes (G. fujikuroi and G. zeae) associated with maize using a seed health blotter test, and the fumonisin levels, using high performance liquid chromatography, in samples of maize grains grown across 23 municipalities during the 2008/09 and 2009/10 growing seasons. Additionally, 104 strains that were representative of all of the analysed samples were identified to species using PCR assays. The mean FDK was seven per cent, and only six of the samples had levels greater than six per cent. Fusarium spp. of the G. fujikuroi complex were present in 96% of the samples, and G. zeae was present in 18% of the samples (5/27). The mean incidence of G. fujikuroi was 58%, and the incidence of G. zeae varied from 2 to 6%. FB₁ was found in 58.6%, FB₂ in 37.9%, and both toxins in 37.9% of the samples. The FB₁ and FB₂ levels were below the quantification limits for 41.3% of the samples, and the mean FB₁ levels (0.66 μg/g) were higher than the mean FB₂ levels (0.42 μg/g). The PCR identification separated the 104 isolates into three of the G. fujikuroi complex: F. verticillioides (76%), F. subglutinans (4%) and F. proliferatum (2%); and G. zeae (anamorph = F. graminearum) (18%). Our results confirmed the dominance of F. verticillioides, similar to other regions of Brazil, but they differed due to the relatively higher incidence of F. graminearum. Total fumonisin levels were below the maximum limit determined by current Brazilian regulations.

A multi-omic systems-based approach reveals metabolic markers of bacterial vaginosis and insight into the disease

BACKGROUND

Bacterial vaginosis (BV) is the most common vaginal disorder of reproductive-age women. Yet the cause of BV has not been established. To uncover key determinants of BV, we employed a multi-omic, systems-biology approach, including both deep 16S rRNA gene-based sequencing and metabolomics of lavage samples from 36 women. These women varied demographically, behaviorally, and in terms of health status and symptoms.

PRINCIPAL FINDINGS

16S rRNA gene-based community composition profiles reflected Nugent scores, but not Amsel criteria. In contrast, metabolomic profiles were markedly more concordant with Amsel criteria. Metabolomic profiles revealed two distinct symptomatic BV types (SBVI and SBVII) with similar characteristics that indicated disruption of epithelial integrity, but each type was correlated to the presence of different microbial taxa and metabolites, as well as to different host behaviors. The characteristic odor associated with BV was linked to increases in putrescine and cadaverine, which were both linked to Dialister spp. Additional correlations were seen with the presence of discharge, 2-methyl-2-hydroxybutanoic acid, and Mobiluncus spp., and with pain, diethylene glycol and Gardnerella spp.

CONCLUSIONS

The results not only provide useful diagnostic biomarkers, but also may ultimately provide much needed insight into the determinants of BV.

Differences between the normal vaginal bacterial community of baboons and that of humans

Humans and baboons (Papio spp.) share considerable anatomical and physiological similarities in their reproductive tracts. Given the similarities, it is reasonable to expect that the normal vaginal microbial composition (microbiota) of baboons would be similar to that of humans. We have used a 16S rRNA phylogenetic approach to assess the composition of the baboon vaginal microbiota in a set of nine animals from a captive facility and six from the wild. Results show that although Gram-positive bacteria dominate in baboons as they do in humans, there are major differences between the vaginal microbiota of baboons and that of humans. In contrast to humans, the species of Gram-positive bacteria (Firmicutes) were taxa other than Lactobacillus species. In addition, some groups of Gram-negative bacteria that are not normally abundant in humans were found in the baboon samples. A further level of difference was also seen even within the same bacterial phylogenetic group, as baboon strains tended to be more phylogenetically distinct from human strains than human strains were with each other. Finally, results of our analysis suggests that co-evolution of microbes and their hosts cannot account for the major differences between the microbiota of baboons and that of humans because divergences between the major bacterial genera were too ancient to have occurred since primates evolved. Instead, the primate vaginal tracts appear to have acquired discrete subsets of bacteria from the vast diversity of bacteria available in the environment and established a community responsive to and compatible with host species physiology.

Comparative genomics of Gardnerella vaginalis strains reveals substantial differences in metabolic and virulence potential

BACKGROUND

Gardnerella vaginalis is described as a common vaginal bacterial species whose presence correlates strongly with bacterial vaginosis (BV). Here we report the genome sequencing and comparative analyses of three strains of G. vaginalis. Strains 317 (ATCC 14019) and 594 (ATCC 14018) were isolated from the vaginal tracts of women with symptomatic BV, while Strain 409-05 was isolated from a healthy, asymptomatic individual with a Nugent score of 9.

PRINCIPAL FINDINGS

Substantial genomic rearrangement and heterogeneity were observed that appeared to have resulted from both mobile elements and substantial lateral gene transfer. These genomic differences translated to differences in metabolic potential. All strains are equipped with significant virulence potential, including genes encoding the previously described vaginolysin, pili for cytoadhesion, EPS biosynthetic genes for biofilm formation, and antimicrobial resistance systems, We also observed systems promoting multi-drug and lantibiotic extrusion. All G. vaginalis strains possess a large number of genes that may enhance their ability to compete with and exclude other vaginal colonists. These include up to six toxin-antitoxin systems and up to nine additional antitoxins lacking cognate toxins, several of which are clustered within each genome. All strains encode bacteriocidal toxins, including two lysozyme-like toxins produced uniquely by strain 409-05. Interestingly, the BV isolates encode numerous proteins not found in strain 409-05 that likely increase their pathogenic potential. These include enzymes enabling mucin degradation, a trait previously described to strongly correlate with BV, although commonly attributed to non-G. vaginalis species.

CONCLUSIONS

Collectively, our results indicate that all three strains are able to thrive in vaginal environments, and therein the BV isolates are capable of occupying a niche that is unique from 409-05. Each strain has significant virulence potential, although genomic and metabolic differences, such as the ability to degrade mucin, indicate that the detection of G. vaginalis in the vaginal tract provides only partial information on the physiological potential of the organism.

Nanoscale periodicity in stripe-forming systems at high temperature: Au/W(110)

We observe using low-energy electron microscopy the self-assembly of monolayer-thick stripes of Au on W(110) near the transition temperature between stripes and the nonpatterned (homogeneous) phase. We demonstrate that the amplitude of this Au-stripe phase decreases with increasing temperature and vanishes at the order-disorder transition (ODT). The wavelength varies much more slowly with temperature and coverage than theories of stress-domain patterns with sharp boundaries would predict, and maintains a finite value of about 100 nm at the ODT. We argue that such nanometer-scale stripes should often appear near the ODT.

Labyrinthine island growth during Pd/Ru(0001) heteroepitaxy

Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small sections of the atomic step edges surrounding Pd islands. This causes a novel epitaxial growth mode in which islands advance in a snakelike motion, giving rise to labyrinthine patterns. Based on density functional theory together with scanning tunneling microscopy and low energy electron microscopy we propose that this growth mode is caused by a surface alloy forming around growing islands. This alloy gradually reduces step attachment rates, resulting in an instability that favors adatom attachment at fast advancing step sections.

[Long-term results following mastoid obliteration in canal wall down tympanomastoidectomy]

BACKGROUND

The primary goal in chronic otitis media is the creation of a dry, safe ear with hearing preservation and reconstruction. This study was designed to evaluate the authors' experience using autologous bone pate and conchal cartilage for mastoid obliteration in canal wall down tympanomastoidectomy.

METHODS

A retrospective long-term review in 2003 of thirty-nine ears was performed, which underwent mastoid obliteration in canal wall down tympanomastoidectomy between 1993 and 2002 performed by the senior author. Data included questionnaire results, clinical observation as well as pre- and postoperative audiometry.

RESULTS

Long-term follow-up shows, that 38 out of 39 ears maintained a small, dry, healthy mastoid cavity. Recurrent cholesteatoma occurred in a single patient. A self-cleaning mastoid bowl was achieved in 90 %, resulting in reduced follow-up visits. Postoperative vertigo in canal wall down revision surgery is clearly diminished due to the mastoid obliteration. Overall, 92 % of patients are satisfied with the surgical result.

CONCLUSION

Obliteration of a canal wall down mastoid cavity by autologous bone pate and conchal cartilage is a reliable and effective technique that results in a dry, trouble free mastoid cavity in 90 % of the patients with chronic otitis media.

[Impact of TORP diameter on fracture of the footplate]

BACKGROUND

A columella-prosthesis is the typical reconstruction procedure of the ossicular chain in cases with a destroyed stapedial arch. Lightweight titanium prostheses are commonly used for this type of surgery. The smaller the prosthesis foot the lower the risk of dislocation and friction of the prosthesis with the bony rim of the oval niche. However, using a smaller prosthesis foot, fracture of the footplate and penetration of the prosthesis into the inner ear are conceivable.

METHODS

Using fresh temporal bone preparations, lacking the stapedial arch, in conjunction with a load cell, the force needed to fracture the footplate was measured. Different prostheses with different foot diameters (0.2 - 0.8 mm) were used in these experiments. Furthermore, a human stapes was evaluated morphologically by scanning electron microscopy.

RESULTS

The forces leading to fracture of the footplate were measured to be between 250 mN and 980 mN. Statistical analysis showed no significant difference between 0.2 mm and 0.8 mm prosthesis foots. Moreover, different forces were attributed to varying thickness of the stapes footplate. In scanning electron microscopy the rough and uneven bony surface of the footplate could be demonstrated.

CONCLUSION

Due to the uneven footplate, only punctual contact of the prosthesis foot can be achieved. Hence, the risk of footplate fracture is not considerably higher by using smaller diameters. However, the use of smaller feet in upcoming generations of TORPs seems to be appropriate.

Critical role of surface steps in the alloying of Ge on Si(001)

Using low-energy electron microscopy, we show that intermixing of Ge on Si(001) during growth is enhanced on stepped surfaces and is hindered on terraces where step flow does not occur. On large terraces we have identified a dramatic and unanticipated structural rearrangement that facilitates intermixing: Pairs of steps spontaneously form and migrate over the surface, leaving alloyed regions in their wake. The driving force for step formation is the entropy gain associated with the enhanced intermixing of Ge.

Changing the diffusion mechanism of Ge-Si dimers on Si(001) using an electric field

We change the diffusion mechanism of adsorbed Ge-Si dimers on Si(001) using the electric field of a scanning tunneling microscope tip. By comparing the measured field dependence with first-principles calculations we conclude that, in negative field, i.e., when electrons are attracted towards the vacuum, the dimer diffuses as a unit, rotating as it translates, whereas, in positive field the dimer bond is substantially stretched at the transition state as it slides along the substrate. Furthermore, the active mechanism in positive fields facilitates intermixing of Ge in the Si lattice, whereas intermixing is suppressed in negative fields.

DPOAE-patterns in different types of autosomal-dominant nonsyndromal hearing impairment

Twelve families with autosomal-dominant nonsyndromal hearing impairment (ADNSHI) were examined. The mode of inheritance was determined by pedigree and at least three generations with affected persons had to be found. Pure tone audiogram (PTA), DP-gram and caloric vestibular test (CVT) were performed on 30 affected persons. By PTA we could find eight families with mild to moderate mid-frequency U-shaped ADNSHI, three families with moderate to severe gently sloping high tone ADNSHI and one family with variable ADNSHI. The corresponding DP-grams showed a family-specific DP-gram in 20 (66%) of the examined persons. Seven (23%) showed no distortion-product otoacoustic emissions (DPOAE). In three (10%) persons the DP-grams varied but were also abnormal. The CVT was normal in all cases. Obviously it is possible to find out typical DP-grams in families with ADNSHI. This could be used for early diagnosis of hearing disorders in newborns of such families. Problems could only occur in progredient cases.

Endemic Trypanosoma cruzi infection in Indian populations of the Gran Chaco territory of South America: performance of diagnostic assays and epidemiological features

The relative specificities and sensitivities of several serological assays for the diagnosis of Trypanosoma cruzi infection were estimated in Indian populations of Argentina and Paraguay. The results obtained with the assays, which proved to be most reliable, were used to study the distribution of the parasite in these populations. Serological evidence of T. cruzi infection was demonstrated in 256 (37.7%) of 679 Indians living in relatively small and isolated communities in the Salta province of northern Argentina and in western Paraguay, regions that are part of the tropical territory called Gran Chaco. In contrast, none of the 94 Indians examined in south-western Argentina was positive. Infection in the Gran Chaco Indians increased with age and clustered in families. Marked differences in seroprevalence were observed between the 16 Indian communities examined in Gran Chaco. These differences seem to be associated both with the risk of transmission from the sylvatic reservoirs of the parasite and with the frequency with which vector-spraying campaigns have been implemented.

Studies on the induction of histocompatibility gene mutations in germ cells of mice by chemical mutagens and/or virus-inducing compounds

This work continues earlier studies concerning the use of histocompatibility mutations in mammalian germ cells as a mutagenicity test system (H test). The rate of spontaneous H mutations was re-examined using a new basis for the classification of H mutants. This procedure led to very high frequencies of suspected spontaneous H mutants: among C57Bl/6 mice, 6% and among C3H mice, 9%. F2 hybrids of a cross between these strains revealed 1% suspected H mutants. Using the same procedure, the sensitivity of the H test was examined with the mutagens ethylnitrosourea, benzo[a]pyrene, 2-acetylaminofluorene (2-AAF), with the solvent dimethyl sulfoxide (DMSO) and with the antibacterial nitrofurantoin. It was possible to demonstrate the mutagenic potential of all mutagens tested as well as their specific action on the different stages of male germ cell development. We succeeded in demonstrating the mutagenicity of 2-AAF for the first time in germ cells of a mammal. In contrast to the negative result with benzopyrene (BP) in the specific locus test, BP induced H mutants even at the very low dose of 2 mg/kg. DMSO was found to induce H mutations in spermatogonia. This extraordinary result is possibly due to the virus-inducing properties of this compound. Nitrofurantoin which is often used in treating bacterial infections of the urinary tract in humans showed a very stage-specific action on maturing spermatids. The value of the H test for mutagenicity testing is discussed with respect to its sensitivity and economy. The very high spontaneous frequency of suspected H mutants and the ease of inducing increased mutant frequencies by mutagens and by DMSO suggest the possibility that the majority of the histoincompatibilities found in the H test are due to induced antigenic gene products of endogenous viruses. This, however, does not interfere with the applicability of the H test for mutagenicity testing, but rather seems to augment its sensitivity to alkylating mutagens as well as mutagens which probably cause frameshift mutations. Other tests for mutations and/or inherited tumor proneness using mouse germ cells can easily be combined with the H test, because the test animal does not have to be killed, thus reducing the cost of the test.

Histocompatibility gene mutation rates, spontaneous and induced by the chemical mutagen procarbazine

The use of histocompatibility mutations in mice for the development of a mutagenicity test has been proposed by several immunologists. The aim of our work was to find a basis for the establishment of the H-test as a mutagenicity test. We therefore determined the spontaneous mutation rates of H-genes in the two inbred mouse strains C3H and C57Bl. Furthermore, we tried to increase the mutation rate by the well-known chemical mutagen procarbazine. The spontaneous mutation rates of H-genes of both strains were identical at about 1.2 x 10(-3). After long-term mutagen treatment with 100 mg procarbazine/kg per week, the mutant frequency increased to about 7% and decreased again when the total dose had reached more than 9 x 100 mg/kg in C57Bl mice and more than 14 x 100 mg/kg in C3H mice. These results are discussed in comparison with procarbazine experiments with other mutagenicity test systems. The feasibility of the H-test for mutagenicity testing remains to be verified by further experiments with other germ-line mutagens.

Suppressor T cells in low zone tolerance. II. Characterization of suppressor T and amplifier cells by physical and serological methods

Suppressor cells involved in low zone tolerance to bacteriophage fd have been characterized by serological and physical methods. Suppressor T cells are peripheral T2 cells which are Ly-1-, Ly-2+ and Ia+. They are of high electrophoretic mobility, sediment 5 mm/h at 1 x g in a linear Ficoll 70 gradient, but are not restricted to a distinct band in a discontinuous bovine serum albumin gradient. Adherent phagocytic cells of low electrophoretic mobility can act as amplifier cells with a much shorter half-life than suppressor T cells.

Suppressor T cells in low zone tolerance. I. Mode of action of suppressor cells

Low zone tolerance (LZT) to bacteriophage fd seems to be a type of tolerance which is primarily caused by suppressor T cells. The aim of this paper is to analyze their mode of action. For the induction of antigen-specific suppressor cells in hydrocortisone pretreated CBA mice, we use tolerogenic and immunogenic doses of antigen. Suppressor activity can be demonstrated upon transfer of spleen cells into normal syngeneic mice. After immunization these animals are unable to produce IgG antibody against phage fd, whereas the IgM response is not suppressed; The half-life of transferred suppressor cells in nonimmunized animals is 5--6 weeks. The target of suppression are unprimed T helper cells, whereas primed helper cells cannot be blocked. T helper cells become "resistant" to suppression 18--36 h after contact with antigen. Differentiation from unprimed B into B memory cells is unaffected, yet under suppression conditions persisting B memory cells are blocked in IgG production. The experimental data are incorporated into a model of LZT.