Characterization of splenic lymphoid cells in fetal and newborn mice

Opportunities, Technology, and the Joy of Discovery

My grandparents were immigrants. My paternal grandfather was illiterate. Yet my parents were able to complete college and to become teachers. I had a conventional upbringing in a small town in Florida, graduating from high school in 1960. I was fortunate enough to graduate cum laude from Florida State University and to earn other credentials leading to faculty positions at outstanding institutions of higher education: the University of Chicago and Northwestern University. At a time when women were rarely the leaders of research groups, I was able to establish a well-funded research program and to make contributions to our understanding of viral entry into cells. My best research was done after I became confident enough to seek productive interactions with collaborators. I am grateful for the collaborators and collaborations that moved our field forward and for my trainees who have gone on to successes in many different careers. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Flt3 ligand treatment reduces enterovirus A71 lethality in mice with enhanced B cell responses

Enterovirus A71 (EV-A71) infection can induce encephalitis, which causes death or long-term neurological sequelae, especially in young children. Using a murine infection model, we searched for anti-EV-A71 agents, because effective therapies are not available to control fatal infection. In EV-A71-infected mice, treatment with the hematopoietic growth factor, Fms-like tyrosine-kinase 3 ligand (Flt3 ligand) before infection reduced the lethality and tissue viral loads. Flt3 ligand failed to enhance the production of type I interferons. Instead, Flt3 ligand boosted the numbers of dendritic cells and, particularly lymphocytes in infected organs with an expansion of spleen B cells, and resulted in an increased titer of virus-specific antibody with neutralizing activity in the serum. The protective effect of Flt3 ligand was abolished in B cell-deficient mice. Our findings revealed that Flt3 ligand administration promotes resistance to EV-A71 infection with enhanced B cell response in a mechanism rarely reported before.

Tracking migration during human T cell development

Specialized microenvironments within the thymus are comprised of unique cell types with distinct roles in directing the development of a diverse, functional, and self-tolerant T cell repertoire. As they differentiate, thymocytes transit through a number of developmental intermediates that are associated with unique localization and migration patterns. For example, during one particular developmental transition, immature thymocytes more than double in speed as they become mature T cells that are among the fastest cells in the body. This transition is associated with dramatic changes in the expression of chemokine receptors and their antagonists, cell adhesion molecules, and cytoskeletal components to direct the maturing thymocyte population from the cortex to medulla. Here we discuss the dynamic changes in behavior that occur throughout thymocyte development, and provide an overview of the cell-intrinsic and extrinsic mechanisms that regulate human thymocyte migration.

Brain development in rodents and humans: Identifying benchmarks of maturation and vulnerability to injury across species

Hypoxic-ischemic and traumatic brain injuries are leading causes of long-term mortality and disability in infants and children. Although several preclinical models using rodents of different ages have been developed, species differences in the timing of key brain maturation events can render comparisons of vulnerability and regenerative capacities difficult to interpret. Traditional models of developmental brain injury have utilized rodents at postnatal day 7-10 as being roughly equivalent to a term human infant, based historically on the measurement of post-mortem brain weights during the 1970s. Here we will examine fundamental brain development processes that occur in both rodents and humans, to delineate a comparable time course of postnatal brain development across species. We consider the timing of neurogenesis, synaptogenesis, gliogenesis, oligodendrocyte maturation and age-dependent behaviors that coincide with developmentally regulated molecular and biochemical changes. In general, while the time scale is considerably different, the sequence of key events in brain maturation is largely consistent between humans and rodents. Further, there are distinct parallels in regional vulnerability as well as functional consequences in response to brain injuries. With a focus on developmental hypoxic-ischemic encephalopathy and traumatic brain injury, this review offers guidelines for researchers when considering the most appropriate rodent age for the developmental stage or process of interest to approximate human brain development.

Reciprocal regulation of Rag expression in thymocytes by the zinc-finger proteins, Zfp608 and Zfp609

Recombination-activating gene 1 (Rag1) and Rag2 enzymes are required for T cell receptor assembly and thymocyte development. The mechanisms underlying the transcriptional activation and repression of Rag1 and Rag2 are incompletely understood. The zinc-finger protein, Zfp608, represses Rag1 and Rag2 expression when expressed in thymocytes blocking T-cell maturation. Here we show that the related zinc-finger protein, Zfp609, is necessary for Rag1 and Rag2 expression in developing thymocytes. Zfp608 represses Rag1 and Rag2 expression indirectly by repressing the expression of Zfp609. Thus, the balance of Zfp608 and Zfp609 plays a critical role in regulating Rag1 and Rag2 expression, which may manifest itself not only during development of immature thymocytes into mature T cells but also in generation of the T-cell arm of the adaptive immune system, which does not fully develop until after birth.

Assessing the health effects and risks associated with children's inhalation exposures--asthma and allergy

Adults and children may have different reactions to inhalation exposures due to differences in target tissue doses following similar exposures, and/or different stages in lung growth and development. In the case of asthma and allergy both the developing immune system and initial encounters with common allergens contribute to this differential susceptibility. Asthma, the most common chronic childhood disease, has significant public health impacts and is characterized by chronic lung inflammation, reversible airflow obstruction, and immune sensitization to allergens. Animal studies described here suggest that air pollutants exacerbate asthma symptoms and may also play a role in disease induction. Changes characteristic of asthma were observed in rhesus monkeys sensitized to house dust mite antigen (HDMA) as infants and exposed repeatedly thereafter to ozone (O3) and HDMA. O3 exposure compromised airway growth and development and exacerbated the allergen response to favor intermittent airway obstruction and wheeze. In Brown Norway rats a variety of air pollutants enhanced sensitization to HDMA such that symptoms elicited in response to subsequent allergen challenge were more severe. Although useful for assessing air pollutants effects on initial sensitization, the rodent immune system is immature at birth relative to humans, making this model less useful for studying differential effects between adults and children. Because computational models available to address children's inhalation exposures are limited, default adjustments and their associated uncertainty will continue to be used in children's inhalation risk assessment. Because asthma is a complex (multiple genes, phenotypes, organ systems) disease, this area is ripe for systems biology approaches.

Control of thymocyte development and recombination-activating gene expression by the zinc finger protein Zfp608

The products of recombination-activating gene 1 (Rag1) and Rag2 are required for T cell receptor gene assembly and thymocyte maturation, yet their transcriptional control mechanisms remain unclear. A congenic strain (called 'ZORI' here) with defects in Rag1 and Rag2 expression, thymocyte maturation and peripheral T cell homeostasis has been developed. Here, we mapped the mutation in this strain to a chromosome 18 locus containing a single known gene encoding the zinc finger protein Zfp608. This gene (Zfp608) was highly expressed in neonatal thymus but was extinguished thereafter. In contrast to wild-type mice, ZORI mice had sustained thymocyte expression of Zfp608 throughout life. The ZORI mutation produced a thymocyte-intrinsic developmental defect. Overexpression of Zfp608 in BALB/c thymocytes substantially impaired Rag1 and Rag2 expression, indicating the underlying mechanism for the defect in ZORI thymocyte development. Thus, the normal function of Zfp608 may be to prevent Rag1 and Rag2 expression in utero.

Species comparison of anatomical and functional immune system development

The components of the immune system have not been traditionally emphasized as potential target organs in standard developmental and reproductive toxicity (DART) protocols. A number of workshops have been organized in recent years to examine scientific questions that underlie developmental immunotoxicity tests, and the interpretation of results as they relate to human risk assessment. A key question that must be addressed is to determine the most appropriate species and strains to model the developing human immune system. The objective of this review is to compare the anatomical and functional development of the immune system in several species important to either preclinical studies for drug development or safety assessments for chemicals, with what is known in humans. The development of the immune system in humans will be compared to what is known in mice, rats, dogs and nonhuman primates.

Flt3 ligand-treated neonatal mice have increased innate immunity against intracellular pathogens and efficiently control virus infections

Flt-3 ligand (FL), a hematopoetic growth factor, increases the number of dendritic cells (DCs), B cells, and natural killer cells in adult mice but the effect in neonates was unknown. We show that FL treatment of newborn mice induced a >100-fold increase in the innate resistance against infection with herpes simplex virus type 1 and Listeria monocytogenes. This resistance required interferon (IFN)-alpha/beta for viral and interleukin (IL)-12 for bacterial infections. Long-term survival after viral but not bacterial infection was increased approximately 100-fold by FL treatment. After treatment, CD11c(+)/major histocompatibility complex type II(+) and CD11c(+)/B220(+) DC lineage cells were the only cell populations increased in the spleen, liver, peritoneum, and skin. DC induction was independent of IFNs, IL-2, -4, -7, -9, -15, and mature T and B cells. The data suggest that FL increases the number of DCs in neonates and possibly in other immune-compromised individuals, which in turn improves IFN-alpha/beta- and IL-12-associated immune responses.

Development of T-B cell collaboration in neonatal mice

The neonatal immune response is impaired during the first weeks after birth. To obtain a better understanding of this immaturity, we investigated the development of T cell interactions with B cells in mice. For this purpose, we analyzed the immune response to three T-dependent antigens in vivo: (i) the polyclonal antibody response induced by vaccinia virus; (ii) the production of polyclonal and specific antibodies following immunization with hapten-carrier conjugates; (iii) the mouse mammary tumor virus superantigen (sAg) response involving an increase in sAg-reactive T cells and induction of polyclonal antibody production. After vaccinia virus injection into neonates, the polyclonal antibody response was similar to that observed in adult mice. The antibody response to hapten-carrier conjugates, however, was delayed and reduced. Injection with sAg-expressing B cells from neonatal or adult mice allowed us to determine whether B cells, T cells or both were implicated in the reduced immune response. In these sAg responses, neonatal T cells were stimulated by both neonatal and adult sAg-presenting B cells but only B cells from adult mice differentiated into IgM- and IgG-secreting plasma cells in the neonatal environment in vivo. Injecting neonatal B cells into adult mice did not induce antibody production. These results demonstrate that the environment of the neonatal lymph node is able to support a T and B cell response, and that immaturity of B cells plays a key role in the reduced immune response observed in the neonate.

Immunologic memory induced by a glycoconjugate vaccine in a murine adoptive lymphocyte transfer model

We have developed an adoptive cell transfer model in mice to study the ability of a glycoprotein conjugate vaccine to induce immunologic memory for the polysaccharide moiety. We used type III capsular polysaccharide from the clinically relevant pathogen group B streptococci conjugated to tetanus toxoid (GBSIII-TT) as our model vaccine. GBS are a major cause of neonatal infections in humans, and type-specific antibodies to the capsular polysaccharide protect against invasive disease. Adoptive transfer of splenocytes from mice immunized with the GBSIII-TT conjugate vaccine conferred anti-polysaccharide immunologic memory to naive recipient mice. The transfer of memory occurred in a dose-dependent manner. The observed anamnestic immune response was characterized by (i) more rapid kinetics, (ii) isotype switching from immunoglobulin M (IgM) to IgG, and (iii) 10-fold-higher levels of type III-specific IgG antibody than for the primary response in animals with cells transferred from placebo-immunized mice. The adoptive cell transfer model described in this paper can be used for at least two purposes: (i) to evaluate conjugate vaccines with different physicochemical properties for their ability to induce immunologic memory and (ii) to study the cellular interactions required for an immune response to these molecules.

Neonatal and early life immune responses to various forms of vaccine antigens qualitatively differ from adult responses: predominance of a Th2-biased pattern which persists after adult boosting

Induction of neonatal immune responses to vaccine antigens is believed to be of limited efficacy because of immune immaturity and particular susceptibility to tolerogenic signals during this period of life. To characterize particular features of neonatal immune responses to vaccine antigens, we assessed the capacity of BALB/c mice at different stages of immunological maturation to respond to a selection of vaccine antigens and presentation systems. Significant B and T cell responses to vaccine antigens (tetanus and measles virus peptides, tetanus toxoid, live viral attenuated measles virus, canarypox recombinant measles vector or bacillus Calmette-Guérin) were obtained as early as the first week of life. However, these neonatal responses differed qualitatively from adult responses by a decreased IgG2a/IgG1 ratio of vaccine-specific antibodies, the secretion of significantly higher interleukin-5 and lower interferon-gamma levels by vaccine-specific T cells and an impaired induction of cytotoxic T cell precursors. This pattern of biased Th2 versus Th1 responses induced upon early exposure to vaccines was not reversed by decreasing the doses of vaccine antigens. It did not disappear with aging and was still reflected in adult responses to booster immunization with the corresponding antigen. Thus, neonatal immunization can induce significant vaccine specific responses with a predominance of a Th2 pattern which can persist in boosted adult mice.

Flow cytometric analysis of the expression of murine B and T surface markers from birth to adulthood

The proportion of nucleated splenocytes bearing B-lymphocyte markers B220, surface IgM (sIgM) and sIgD, as well as the T-lymphocyte markers Thy 1.2, CD5, CD8a and CD4 were quantitated by flow cytometric analysis (FACS) throughout postpartum development in the A/J mouse. Full expression of B lymphocyte markers was achieved much sooner than expression of T lymphocyte markers. This was especially true for B220, which was found on 8% of all splenocytes at day 5 and reached adult levels (47-50%) by weaning at day 22. Expression of sIgM and sIgD were 13% and 9%, respectively, of all splenocytes at day 5 with mature levels not expressed until day 35 postpartum (approximately 36% of cells were positive for these markers). T lymphocyte markers, on the other hand, did not reach full expression until sexual maturity. For example, Thy 1.2 expression was 8% on day 5 and did not reach mature levels (28-30%) until day 56. CD5 closely paralleled Thy 1.2 expression rising from only 2% on day 5 to 27% by day 56. Likewise, CD8a and CD4 marker development paralleled one another with CD8a rising from 1% on day 5 to 10% by day 56 and CD4 rising from 5% on day 5 to 19% by day 56. These data demonstrate the variability in the time of appearance and rate of maturation of the various lymphocyte cell surface markers during postpartum development. They also serve as a reference to identify alterations in lymphocyte development created by immunodeficiency diseases.

Background (spontaneous) immunoglobulin production in the murine small intestine as a function of age

The development of the intestinal B-cell compartment in C3H/He mice was studied as a function of age by quantification of the number of intestinal immunoglobulin-secreting cells (Ig-SC). Before and at weaning, the number of Ig-SC in the small intestine (SI) was below 10(3) Ig-SC per SI. During the first few weeks after weaning, this number rose steeply and continued to rise until the mice were about 48 weeks old, when a maximum of more than 25 x 10(6) Ig-SC per SI was found. After 1 year of age the number of Ig-SC decreased. At all ages, the great majority of Ig-SC in the SI produced IgA. The increase of the number of IgA-SC in the SI after weaning is reflected in the amount of IgA in intestinal secretions measured by ELISA. The number of Ig-SC in the SI was compared with the number of Ig-SC found in spleen, bone marrow, Peyer's patches and mesenteric lymph nodes. Striking differences were observed between the SI and the other organs tested in total number, isotype distribution and kinetics of the increase of Ig-SC during ontogeny. These differences are discussed in relation to the regulation of the immune response in the SI and the migration patterns of lymphocytes in mucosal tissues.

Strain differences in the postnatal development of the mouse splenic lymphoid system

Four strains of mice, CBA/J, BALB/c, C57BL/6J and B10.BR, were studied for cellular composition of the developing spleen in postnatal life. Considerable strain differences were found in the absolute numbers of splenic lymphoid cells at various ages, the frequencies of B and T cells, the L3T4/Lyt-2 ratio and the time of appearance of IgD and class II antigen on B cells. These observations are discussed in view of strain differences described for the acquisition of immune responsiveness and for susceptibility to tolerance induction in neonates. Finally, it was found that the expression of I-E antigen is delayed compared to that of I-A in ontogeny and possibly during B-cell differentiation as well.

Analysis of delta mRNA in the immature B lymphocytes of neonatal liver and spleen

The production of mu m and delta m mRNA was assessed by measuring the relative levels of the two species in nuclear RNA isolated from neonatal liver and from spleen cells obtained from mice of differing ages. These analyses showed that the lack of IgD expression in the immature B cells of neonatal liver is due to an absence of delta mRNA. The presence of high mol. wt species which hybridize to a delta probe indicates that the delta coding regions are transcribed and suggest that post-transcriptional events account for the lack of delta expression. Little change in the mu m-delta m mRNA ratio was observed in nuclear RNA isolated from spleen cells of mice aged from 5 days to 4 months. Thus, variations in the levels of mRNA synthesis play an insignificant role in the changes in IgM and IgD expression seen during B cell maturation in neonatal spleen.

Murine neonatal spleen contains natural T and non-T suppressor cells capable of inhibiting adult alloreactive and newborn autoreactive T-cell proliferation

The spleen of neonatal mice is known to be a rich source of cells capable of suppressing a variety of immune functions of adult lymphocytes in vitro. From such observations has emerged the concept that the gradual development in ability to express immune functions after birth is due in part to the parallel normal physiological decay of naturally occurring regulatory suppressor cells. There is, however, some confusion in the literature as to the exact nature of the newborn of the newborn inhibitory cell type(s). In contrast to most previous reports which detect only a single type of neonatal suppressor cell, usually a T cell, we show here that newborn spleen harbors both T and non-T inhibitory cells. Both types of suppressor cells could be shown to suppress the proliferative response of adult spleen to alloantigens as well as newborn T cells reacting against self-Ia antigen in the autologous mixed lymphocyte reaction (AMLR). Newborn suppressor T cells were characterized as being non-adherent to Ig-anti-Ig affinity columns, soybean agglutinin receptor negative (SBA-), and susceptible to lysis by anti-T-cell specific antiserum plus complement. Non-T suppressor cells were identified as non-phagocytic, SBA receptor positive (SBA+), and resistant to cytotoxic treatment with anti-T-cell antibodies and complement. The apparent controversy surrounding previous reports as to the T versus non-T nature of newborn suppressor cells can be reconciled by the present observation that both types of inhibitory cells coexist in the spleen. Furthermore, the demonstration that newborn suppressor cells can effectively regulate T-cell proliferative activity mediated by other newborn cells provides more direct support for the contention that such inhibitory cells play a physiological role in controlling immune responsiveness during early ontogeny.

Ontogenic development of "natural" and induced plaque-forming cell isotypes in normal mice

The numbers of cells and background plaque-forming cells (PFC) in the spleen of C3H/HeJ mice increase exponentially during the first 2 weeks after birth, but much slower in bone marrow (BM). IgG1 and IgG2a PFC are the first non-IgM PFC detectable, while IgG3 and IgA PFC appear only around weaning. Adult-type PFC numbers and isotype pattern are present in spleen and BM at 4 and 15 weeks, respectively. Neonatal splenic C3H/Tif B cells produce non-IgM Ig classes in vitro in response to polyclonal activation by lipopolysaccharide or by helper T cells. These responses are of low magnitude during the first 2 weeks of life, but both secreted and membranebound IgG1 and IgG3 isotypes are detectable already a few days after birth, in a pattern that is identical to that typical of T cell-dependent or independent responses of adult cells. These results indicate full maturity of B cells in "switch" abilities already from birth, in spite of a general deficiency in terminal maturation. In addition, they demonstrate the complexity of isotype regulation in "background" antibody production in vivo.

During frog ontogeny, PHA and Con A responsiveness of splenocytes precedes that of thymocytes

The in-vitro proliferation of splenocytes and thymocytes from Xenopus laevis-gilli (hybrid clone LG-15) to the T cell mitogens, concanavalin A (Con A) and phytohaemagglutinin-P (PHA), were examined at specific stages of larval development (stages 51-66 of Nieuwkoop & Faber, 1967) and at 2 months post-metamorphosis. The responses of splenic lymphocytes to each mitogen were significant at all stages with stimulation indices ranging from 1.9 to 50.5 and 2.6 to 45.5 for PHA and Con A, respectively. Stage-related differences in responses of splenocytes to both mitogens suggest two waves of emergence of proliferative activity during development, divided by periods of diminished responsiveness during the metamorphic crisis. In contrast to the responses observed with splenocytes, proliferation of thymocytes cultured with either mitogen was barely detectable, with stimulation indices ranging from 1.2 to 6.9 and 1.4 to 2.9 for PHA and Con A, respectively. These minimal responses were observed only when thymocytes were cultured at relatively high cell density (5 X 10(5) cells/ml); they were not improved by increased or decreased concentrations of mitogen or by increased concentrations of fetal calf serum (5 or 10%) in the medium. Co-culture of larval thymocytes with autologous splenocytes and each mitogen did not consistently increase thymocyte responses suggesting that the defect in thymocyte responsiveness is not due to lack of accessory cells. These findings suggest that if PHA- and Con A-reactive cells are present in the thymus, they are present in relatively low numbers at all stages of larval development. The pattern of early mitogen responsiveness in the spleen at a time when the thymus is unresponsive contrasts with that observed in mammalian development in which thymocytes become responsive to mitogens in fetal stages and mitogen responsiveness appears in the spleen only around the time of birth. The apparent inactivity of larval thymocytes may reflect a population of cells that can become tolerant to those neo-self-antigens that arise during and after metamorphosis. If so, the larval amphibian thymus may provide a model to study the early events of thymocyte 'education' and differentiation in a broader time framework than is possible with fetal mammals.

Mode of regulation of immunoglobulin mu- and delta-chain expression varies during B-lymphocyte maturation

The transcription, processing, and accumulation of mu and delta mRNA was studied in several cell lines representing different stages of B-lymphocyte maturation. Our results indicate that the relative content of mu and delta mRNA is the major determinant of the IgM versus IgM + IgD phenotype and that the production of delta mRNA is regulated at distinctly different levels of gene expression in early and later developmental stages. In B cell lymphomas typical of early stages, transcription extends over the entire 25 kb of the mu- delta locus and the relative content of mu and delta mRNA is determined at the level of mRNA processing. In contrast, in cells typical of mature IgM secretors, transcription is terminated abruptly between the mu and delta genes, precluding the production of delta mRNA. We propose a model that accounts for qualitative and quantitative changes in mu-delta expression in the developing B lymphocyte.

Analysis by limiting dilution of interleukin 2-producing T cells in murine ontogeny

The ontogeny of interleukin 2 (IL 2) production in young CBA/HT6T6J mice was studied using both bulk culture and limiting dilution methods. The ability of spleen cells in bulk culture to produce IL 2 in response to concanavalin A (Con A) was found to rise through the first 2 weeks of life, from no production at day 1, through 20 units/ml at day 6, to 80-100 units/ml in adults. No evidence for suppression of IL 2 production by young spleen was found. Limiting dilution analysis of both young spleen and young lymph node (LN) shows that young spleen has a much lower complement of cells producing IL 2 in response to Con A or allogeneic stimulation than does adult spleen. The frequency of 6-day spleen cells producing IL 2 in response to Con A is 1/1000, while the adult frequency is approximately 1/50. Young LN, in contrast, has levels of IL 2-producing cells close to those of adult LN, with a frequency of responders to Con A of 1/20. No evidence was found for a deficiency in IL 2 production on a per cell basis, in either 6-day spleen or LN. In examining allogeneic reactivity, a high frequency of cells reacting to strong Mls stimulation was found in both young and adult spleen and LN.

Formation of antigen specific foci as a complement independent assay for individual antibody-secreting cells

Antibody-forming cells (AFC) could be demonstrated by the binding of antigen to secreted antibody localized around the AFC. Using sheep red blood cells (SRBC) as antigen, this antigen binding was detected as foci of erythrocytes surrounding individual lymphocytes. Focus formation was antigen specific and involved the active secretion of antibody. AFC specific for antigens resistant to lysis, such as human red blood cells were also demonstrable with this assay. The focus assay provides a complement independent method for enumerating antibody-forming cells.

Role of I-J in neonatal suppression

Spleen cells from neonatal mice belonging to strains with the I-J-b or I-J-k genotype, were treated with anti-I-Jb or anti-I-Jk antibody and complement. This reduces their suppressor cell activity as demonstrated by a decrease in the ability to suppress the mixed-lymphocyte reactivity of adult spleen cells. Injection of anti-I-J antibody into neonatal mice also reduces splenic suppressor cell activity prematurely. The removal of suppressor cells from neonatal spleen does not result in an immediate increase in mixed-lymphocyte reactivity (cell-mediated immunity) but does hasten the development of mixed-lymphocyte reactivity in the young mice. The results are discussed in light of the hypothesis that suppressor cells inhibit the function of immunocompetent cells in the neonatal mouse and control the development of immunocompetence.

Isolation and identification of the naturally occurring, newborn spleen-associated suppressor cells. A mixed monocyte/mast cell population with separable suppressor activities

Spleens from newborn mice less than 6-7 days of age contain 'naturally occurring' suppressor cell populations that are able to inhibit nonspecifically immune responses of third-party adult spleen cells and alloreactivity in the newborn spleen per se. Isolation of the effectors of this suppressor activity reveals that they are not classical T lymphocytes but, instead, a mixed population of cells of the monocyte series (monoblast/promonocyte/monocyte) plus mast cells. This mixed population apparently can elicit its suppressor activity in part through secretion of soluble, in vitro culture-stable material, which in turn initiates activation of the suppressor cell/limb of the immune response. These activities, nevertheless, modulate most strongly the early activation events of T-cell responses and can result in complete suppression of alloreactive helper and cytotoxic T lymphocyte development. Suppressor activity by mast cells can be demonstrated by degranulation, whereas suppressor activity by monocytes appears inherent in the newborn population. Thus, there are now at least three cell populations in the neonate spleen--mast cells, monocytes, and T lymphocytes--which can effect suppressor activity.

Maturation of the lymphoid system. IV. Ontogenetic compartmentalization of T cell function

Pretreatment of one day old but not eight day old or adult A/J mice with soluble ovalbumin (OVA) initiated specific T-cell unresponsiveness as reflected both in T-cell dependent cellular proliferation and in anti-hapten antibody responses to dinitrophenyl-OVA. In contrast, injection of soluble human gamma globulin into either neonatal or adult A/J mice resulted in unresponsiveness. The ability of lymph node T-cells to be sensitized by protein antigens occurred shortly after birth, since the degree of sensitization in 9 and 26 day old mice was similar. Finally, a striking ontogenetic difference was noted in the capacity of lymphocytes derived from the lymph nodes and spleens of young mice to respond to T-cell mitogens. Thus, while splenocytes obtained from 9 day old mice exhibited meager responses to PHA and Con A, lymph node cells from these animals responded at nearly adult levels. These observations are interpreted as reflecting an ontogenetic and tissue-specific division of T-cell function.

Development of bronchus associated lymphoid tissue (BALT) in the rat, with special reference to T- and B-cells

The development of bronchus associated lymphoid tissue was studied in normal rats, in serial sections, using both routine histological techniques and the two-step immuno-peroxidase method on cryostat sections for demonstration of T lymphocytes, IgM-, IgG- and IgA-bearing B lymphocytes and plasma cells, respectively. BALT first appears 4 days postpartum (p.p.) as a condensation of reticulum cells near a lymph vessel, between the main bronchus and the accompanying artery. Only a few lymphocytes are present at first. Vascularisation is considerable 8 days p.p. and includes high endothelial venules. Leucocytes are seen in transit in both blood- and lymph vessel walls. Lymphocytes populate the area under the epithelium from about 2 weeks p.p. A few IgM, IgG- and IgA-bearing B cells are already present at 4 days, and rapidly increase in numbers; T cells usually appear at day 8. Discrete T- and B-cell areas do not appear until 4 weeks and are only observed regularly after 12 weeks, when secondary follicles appear. BALT starts development at a similar time to other peripheral lymphoid organs but apparently achieves immunological activity later. It is concluded that antigens probably play an important part in organising BALT.

Differential regulation of membrane and secretory mu chain synthesis in human beta cell lines. Regulation of membrane mu or secreted mu

Regulation of membrane and secretory mu synthesis was examined in human lymphoblastoid cell lines representing various stages of differentiation. Immunoglobulin phenotype was determined by surface and cytoplasmic staining with fluorochrome-conjugated antibodies and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of anti-mu precipitable cellular products. The thymidine analogue, 5-bromo-2'-deoxyuridine (BUdR), which inhibits differentiation-specific proteins in a variety of systems, was used to examine regulation of immunoglobulin synthesis. We found that BUdR had a differential effect on membrane (mum) and secretory (mus) type mu heavy chains. Ig production in pre-B and plasma cell-like lines, which make mus, was unaffected by BUdR. However, surface expression of IgM (mum) in B cell lines was drastically inhibited at similar doses of BUdR without diminishing total Ig or protein synthesis. Examination of labeled mu chains from control and BUdR-treated B cell lines by SDS-PAGE revealed the production of two sizes of mu (mum and mus) in control cells and only the smaller size (mus) in BUdR-treated cells. This size difference could not be attributed to alterations in glycosylation of the molecules. These data show that BUdR inhibits the production of membrane mu chains without diminishing secretory mu chain synthesis in the same cell. Our findings suggest that thymidine-rich regions of the genome are involved in the regulation of mum vs. mus during B cell differentiation.

Immunosuppressor cells from newborn mouse spleen are macrophages differentiating in vitro from monoblastic precursors

Newborn mouse spleen, whose cells strongly suppress the in vitro humoral response of adult spleen cells, is essentially a hematopoietic organ. It contains a large percentage of proliferating cells, among which about 50% are erythroblasts (identified by their spectrin content) and about 15% are cells of the myelocytic and monocytic lineage. Lymphoid cells are a minority, with about 20% B and only 1-2% T lymphocytes. After a 4 days, a culture of newborn spleen cells contains 5-10 times more macrophages than that of an adult spleen. Most of these macrophage precursors from the newborn spleen are proliferating cells, partially glass- or plastic-adherent, which differentiate in culture into activated macrophages producing large amounts of plasminogen activator. It is this macrophage excess which is responsible for the immunosuppressive effect of newborn spleen cells in culture, as indicated by (a) the effect of silica particles added to the cultures, which both relieve the suppression and prevent the accumulation of macrophages and (b) the suppression of the humoral response of adult spleen cells when they are cultured on the adherent cells from a newborn but not from an adult spleen. The suppressive effect of macrophages seems to result, at least in part, from the production of prostaglandin, since it can be relieved by indomethacin or aspirin. Suppression is not related to arginine depletion of the medium or to production of an excess of plasminogen activator. T lymphocytes from newborn spleen or lymph nodes have no suppressive capability.

Thymic function in NZB mice. III. Impairment of the activity of specific suppressor cells involved in the regulation of antibody production against sheep red blood cells

Spleen cells from DBA/2 mice immunized with high numbers of sheep red blood cells specifically suppress the primary anti-SRBC antibody response of syngeneic recipients specifically suppress the primary anti-SRBC antibody response of syngeneic recipients after in vivo transfer. Such suppressive activity of the immune spleen cells is mediated by null cells, or by T cells resistant to the cytotoxic activity of anti-Thy 1.2 antiserum plus complement. The primary anti-SRBC antibody response is much higher in NZB mice than in DBA/2 mice, and the suppressive activity of syngeneic immune spleen cells is much lower in NZB than in DBA/2 recipients. Immune spleen cells from DBA/2 donors do not provide more effective suppression than NZB spleen cells in NZB recipients. Conversely, immune spleen cells from NZB donors strongly suppress the anti-SRBC primary response of DBA/2 recipients to the same extent as DBA/2 immune spleen cells. Finally, NZB mice generate specific suppressor cells but their primary antibody response is not sensitive to this suppressor activity.

Ontogenic development of B-lymphocyte function and tolerance susceptibility in vivo and in an in vitro organ culture system

The maturation of B-lymphocyte function during fetal development was studied in vivo and in an in vitro organ culture system. The results indicated that the progenitors for 2,4-dinitrophenol (DNP)-specific B cells are present as early as 14 d of gestation in liver and possibly as early as 15 d in spleen. In addition, it was found that the organ culture system supports the development of B lymphocytes as measured by an increase in both the percentage of surface immunoglobulin-positive cells and the frequency of clonable DNP-specific B cells after culturing. The majority of anti-DNP-secreting clones resulting from the antigenic stimulation of fetal B cells produced only the IgM isotype, and the ability to secrete the IgG isotypes increased as a function of gestational age. Because fetal DNP precursors from spleens and livers that had been incubated in organ culture resulted in a greater proportion of clones secreting IgG compared with age-matched uncultured controls, it was concluded that the maturation with regard to the ability to secrete IgG can occur in vitro. In studies relating to the ontogenetic development of tolerance susceptibility, it was found that up to one-half of the DNP-specific B-cell precursors from livers and spleens less than 18 or 19 d of gestation were resistant to tolerogen treatment for 24 h as if in a pretolerant phase. However, if tolerogen were present for 3--5 d during organ culture there was near total elimination of potential DNP clones. This finding suggested that the 24-h induction period was insufficient for affecting the DNP-specific precursors in livers and spleens from the earlier gestational ages, and that a proportion of precursors could subsequently form DNP clones in the splenic focus assay after the removal of tolerogen.

The fate of fetal and adult B-cell progenitors grafted into immunodeficient CBA/N mice

The relative ability of various precursors to generate functional B cells in vivo was assessed by transferring normal, chromosomally-marked CBA/H-T6T6 cells to irradiated or unirradiated immunodeficient CBA/N mice. Emergence of donor-derived B cells was monitored by means of a B-cell cloning assay (in which CBA/N cells are inactive), and by karyotpic analysis of lymphoid, myeloid, and stem cell metaphases. Grafts of lymph node, spleen, anti-mu surface immunoglobin suppressed bone marrow, sIg+ cell-depleted marrow, normal marrow, fetal liver, and yolk sac suggest: (a) there is little self-renewal of sIg+ B cells in these models; (b) pre-committed cells have extensive proliferative/differentiative potential and at least initially contribute most of the newly-formed B cells; (c) populations or pre-B cells obtained from various sources differ in their regenerative ability; (d) CBA/N mice are deficient in a category of pre-B cells which are found in fetal liver; and (e) selective B-cell chimerism results from grafting of unirradiated CBA/N mice.

Development of T and B cell areas in peripheral lymphoid organs of the rat

In the present study the early development of peripheral lymphoid organs (spleen, popliteal lymph node, mesenteric lymph node and Peyer's patches) is described in terms of homing patterns of T and B cells, demonstrated with immunohistoperoxidatic detection of characteristic membrane antigen in normal rats and with routine histology in neonatally thymectomized rats. In the first days after birth the peripheral lymphoid organs are almost exclusively populated by T cells. After neonatal thymectomy lymphocytes appear in the dome areas of Peyer's patches from four to six days after birth, in mesenteric and popliteal lymph nodes lymphocytes are found in the outer cortex from day 6 and day 8 respectively and in the marginal zone of the spleen from eight days onwards. These lymphocytes showed no membrane staining when reacted for T antigen with immunohistoperoxidatic techniques. The morphological evidence for considering Peyer's patches of rats as central inductive sites for the generation of B cells is poor. The discrepancy in the order of appearance of T and B cell (sub)populations in spleen compartments in normal ontogenetic development and lethally irradiated, stem cell reconstituted animals is discussed.

Surface immunoglobulin on cultured foetal mouse thymocytes

Organ cultures of 14–15 day foetal mouse thymus were used as a source of non-neoplastic differentiating T cells, free of contaminating B cells. Viable cells obtained from such cultured thymuses were radio-iodinated and immunoglobulins (Ig) were isolated by co-precipitation from the ¹²⁵I-labelled cell-surface proteins released during 1 h of incubation at 37°. The precipitates, both reduced and unreduced, were then analysed by polyacrylamide gel electrophoresis. The unreduced material migrated in a 5% gel as a single peak with a mobility slightly faster than that of mouse IgG. After reduction, however, two peaks were obtained (in a 10% gel), one corresponding in migration to mouse light chain and the other which moved slightly faster than mouse μ chain. This pattern was identical with that previously seen for both surface Ig of normal mouse thymocytes and neoplastic T lymphoma cells. Uncultured, 15 day foetal thymocytes did not produce any detectable co-precipitated cell surface material. Ig detected in these experiments was therefore produced during in vitro culture by non-neoplastic T cells in a system free of contaminating B cells and mouse serum proteins.

Adult and pre-adult thymectomy of mice: contrasting effects on immune responsiveness, and on numbers of mitogen-responsive and Thy-1+ lymphocytes

Peripheral lymphoid tissues of mice which have been thymectomized at 2 or 4 weeks of age, that is, before they achieve adult body weight, have been shown to be lacking in cells responsive to the T-cell mitogen, phytohaemagglutinin, when the animals became adult, and these mice have also been shown to have a deficient immune response against sheep erythrocytes. It is suggested these effects of pre-adult thymectomy are consequent upon removal of the prime source of T cells prior to the animal having acquired complete T-cell populations of the adult. Spleens and lymph nodes of mice thymectomized at 8 weeks of age were found to have reduced numbers of cells susceptible to the cytotoxic effects of anti-Thy-1 serum as early as 4 weeks after the operation, whereas the number of lymphocytes responsive to T-cell mitogens in these lymphoid tissues was not reduced at this time. The number of spleen-borne antibody-producing cells in a primary or secondary response was not affected by 8-week thymectomy, either when the response was tested in the operated animal, or after transfer of cells from such an animal to an irradiated recipient. The results are discussed with respect to other work on the effects of thymectomy of mice during the post-neonatal and pre-adult period.