Demonstration of large-scale migration of cortical thymocytes to peripheral lymphoid tissues in cyclosporin A-treated rats.

Young adult Lewis rats were maintained on diets containing 0.015 or 0.027% cyclosporin A (CSA) for periods of up to 6 wk. All animals showed complete depletion of medullary thymocytes (CD4+8- and CD4-8+, T cell receptor [TCR] alpha/beta hi, Thy-1med/low, terminal deoxynucleotidyl transferase negative [TdT-]) and a 50% reduction in the number of TdT- cortical thymocytes (CD4+8+, TCR alpha/beta low, Thy-1med) within 1 wk of CSA treatment. In addition, about half of the animals displayed a 50% reduction in the number of TdT+ cortical thymocytes (CD4+8+, TCR alpha/beta low, Thy-1hi). These intrathymic changes were accompanied by a reciprocal increase in the number of double-positive (DP; CD4+8+) T cells in lymph nodes (LN) and spleens. To confirm that the latter T cells were recent thymic emigrants (RTE), CSA-treated rats were injected intrathymically with fluorescein isothiocyanate, and the phenotype of the labeled T cells appearing in LN was determined 16 h later. The results demonstrated that, in addition to those RTE exported in normal animals (> 90% medullary origin), the emigration of DP thymocytes, including large numbers of TdT+ thymocytes, was markedly increased. The presence of TdT+ cells, which normally do not leave the thymus, clearly identifies the DP RTE as originating from the thymus cortex. Intrathymic labeling studies also directly demonstrated that export of all thymocyte subsets ceases within 9 d of CSA treatment; and thymectomy experiments confirmed that the CSA-induced increase in phenotypically immature T cells resulted primarily from the disturbance of thymocyte maturation and emigration, rather than from a direct effect on preexisting T cells. These results suggest that a wave of cortical thymocytes, many of which presumably have not yet undergone negative selection, is released from the thymus during the first week of CSA treatment. The presence of these potentially unselected cells in peripheral lymphoid tissues may help to explain the increased frequency of autoreactive T cells observed in CSA-treated animals.

Migration of lymphocytes and thymocytes in the rat. I. The route of migration from blood to spleen and lymph nodes.

The cellular deficit in rats thymectomized at birth is primarily one of circulating small lymphocytes. The lymphocyte deficiency is similar to that induced in adult rats by chronic drainage from a thoracic duct fistula. In both cases, the animals show a reduction of small lymphocytes in peripheral blood, thoracic duct lymph, and in circumscribed areas of lymphoid tissue. The lympocyte deficiency in lymphoid tissue can be corrected by an intravenous injection of thoracic duct lymphocytes. The evidence suggests that the deficiency is corrected by small lymphocytes. Small lymphocytes pass from blood to lymphoid tissue along a route which includes the marginal sinus in splenic white pulp and postcapillary venules in the cortex of lymph nodes and Peyer's patches. Neither the ability of small lymphocytes to colonize lymphoid tissue nor their ability to traverse postcapillary venules are thymus-dependent phenomena. However, movement of small lymphocytes across postcapillary venules appears to modify the structure of endothelium. Intravenously injected small thymocytes migrate to lymphoid tissue in smaller numbers than small lymphocytes inoculated by the same route. The few thymocytes which localize in lymphoid tissue follow the same pathway as circulating small lymphocytes.

The serum bactericidal system: ultrastructural changes in Neisseria meningitidis exposed to normal rat serum.

Exposure of meningococci to the bactericidal system of normal rat serum initiates a series of ultrastructural changes that accompany death of the organism. These morphological alterations consist of complement-dependent holes in the cell wall outer membrane, edema of the periplasmic space and cytoplasm, and accumulation of fibrillar material in and on the cell wall. Dissolution of the dense line and rupture of the cytoplasmic membrane also occur in meningococci exposed to normal rate serum. These latter two changes, however, are not seen in meningococci that are killed by bentonite-absorbed (lysozyme-deficient) serum, nor are they invariably present when other serum-susceptible organisms (N. catarrhalis and Herellea sp.) are treated with normal rat serum. The pattern of development of edema in serum-treated meningococci suggests that the cell wall outer membrane, mucopeptide layer, and cytoplasmic membrane act synergistically to maintain osmotic equilibrium of the bacterium. Death of the bacterium seems related to alteration of permeability following injury to the outer membrane. Holes are demonstrable, by negative straining, in the outer membrane of the meningococcal cell wall after exposure to the bactericidal effects of rat serum. The lesions are 110 A average greatest diameter and depend on the presence of antibody and complement for their formation. In addition, 82 A diameter holes are present in the walls of normal, untreated meningococci. The relation of complement-dependent holes to the smaller, naturally occurring holes is not known.

Immunoglobulin molecules on the surface of activated T lymphocytes in the rat.

Lewis strain rat lymphocytes were exposed in vitro to a variety of specific and nonspecific blastogenic stimuli. The surfaces of the transformed lymphocytes were examined by indirect immunofluorescence for the presence of T cell antigens and immunoglobulin molecules. More than 90% of lymphocytes that underwent blast transformation after exposure to foreign histocompatibility antigens (mixed lymphocyte reaction; in vitro allograft reaction), purified tuberculin, phytohemagglutinin (PHA), and concanavalin A (Con-A) had T cell antigenic markers on their surfaces. 70-92% of the antigen-stimulated blast cells also had readily detectable surface immunoglobulin molecules, whereas less than 3% of the PHA- and Con-A-activated cells were Ig(+). Pokeweed mitogen (PWM) appeared to activate both B and T cells, but the T cells did not have detectable surface immunoglobulin molecules. Nonactivated control cultures contained T(+)Ig(-) lymphocytes almost exclusively. The results suggest that thymus-dependent rat lymphocytes express increased amounts of detectable immunoglobulin on their surface in response to specific stimulation with antigen. It is postulated that the acquisition of immunological competence by activated T cells may be related to this expression of surface immunoglobulin.

Anatomical distribution of T and B lymphocytes in the rat. Development of lymphocyte-specific antisera.

A method is described whereby antisera raised in rabbits to rat thoracic duct lymphocytes were made specific for the plasma membrane antigens of T and B lymphocytes. These lymphocyte-specific antisera were used in immunofluorescence assays to study the distribution of B and T cells in lymphocyte containing tissues and body fluids of the rat. Rabbit antirat B-cell serum (ALS(B)) reacted selectively with the surfaces of lymphocytes in the lymphoid follicles of lymph node cortex and in the follicles and marginal zones of splenic white pulp, but not with the surfaces of germinal center cells or plasma cells. An identical pattern of fluorescent staining was obtained with rabbit antirat Ig serum. It was shown by blocking, absorption, and immunoprecipitation studies that ALS(B) was composed in large part of antibodies to rat Ig, but that it contained antibodies to other B-cell antigens as well. Rabbit antirat T-cell serum (ALS(T)) reacted selectively with the surfaces of lymphocytes in the paracortex of lymph node and in the periarteriolar sheath of spleen, and with thymocytes. ALS(T) did not display anti-Ig activity. ALS(T) reacted with approximately 100% thymocytes and with 90% thoracic duct, 80% lymph node, 60% blood, 50% spleen, and 10% bone marrow lymphocytes in suspensions of cells from these sources. ALS(B) reacted with the remainder of the lymphocytes in the suspensions, except for bone marrow in which only 59% of lymphocytes had detectable B- or T-cell surface antigens. The population of T lymphocytes in rat bone marrow was depleted by drainage of lymphocytes from a thoracic duct fistula, thereby establishing their membership in the pool of recirculating T cells. Approximately 14% of lymphocytes issuing from the thoracic duct of TxBM donors reacted with ALS(T). The presence in these animals of a small number of T cells, calculated to be approximately 2% of the normal value, may account for the limited capacity of TxBM rats to respond to antigens that induce a cell-mediated immune response.

The importation of hematogenous precursors by the thymus is a gated phenomenon in normal adult mice.

Hematogenous precursors repopulate the thymus of normal adult mice, but it is not known whether this process is continuous or intermittent. Here, two approaches were used to demonstrate that the importation of prothymocytes in adult life is a gated phenomenon. In the first, age-dependent receptivity to thymic chimerism was studied in nonirradiated Ly 5 congenic mice by quantitative intrathymic and intravenous bone marrow (BM) adoptive transfer assays. In the second, the kinetics of importation of blood-borne prothymocytes was determined by timed separation of parabiotic mice. The results showed that >60% of 3-18-wk-old mice developed thymic chimerism after intrathymic injection of BM cells, and that the levels of chimerism (range, 5-90% donor-origin cells) varied cyclically (periodicity, 3 to 5 wk). In contrast, only 11-14% of intravenously injected recipients became chimeric, and chimerism occurred intermittently (receptive period approximately 1 wk; refractory period approximately 3 wk). In the intravenously injected mice, chimerism occurred simultaneously in both thymic lobes; gate opening occurred only after most intrathymic niches for prothymocytes had emptied; and the ensuing wave of thymocytopoiesis encompassed two periods of gating. These kinetics were confirmed in parabiotic mice, and in cohorts of mice in whom gating was synchronized by an initial intrathymic injection of BM cells. In addition, a protocol was developed by which sequential intravenous injections of BM cells over a 3 to 4 wk period routinely induces thymic chimerism in the apparent absence of stem cell chimerism. Hence, the results not only provide a new paradigm for the regulation of prothymocyte importation during adult life, but may also have applied implications for the selective induction of thymocytopoiesis in nonmyeloablated hosts.

Studies of thymocytopoiesis in rats and mice. I. Kinetics of appearance of thymocytes using a direct intrathymic adoptive transfer assay for thymocyte precursors.

We describe a quantitative intrathymic (i.t.) adoptive transfer system for detecting thymocyte precursor cells in rats and mice. In this system, the generation of donor-origin thymocytes is analyzed on the FACS after the injection of test cells directly into the thymus of sublethally irradiated, histocompatible, RT-7 (rat) or Ly-1 (mouse) alloantigen-disparate recipients. Like the standard i.v. adoptive transfer assays for prothymocytes, the i.t. transfer assay is time, dose, and irradiation dependent. However, unlike the i.v. assays, the i.t. assay is highly sensitive, independent of cell migration, and specific for T-lineage precursor cells. Thus, the i.t. system requires between 25- and 50-fold fewer precursor cells than do the i.v. systems to generate a given number of donor-origin thymocytes; it detects nonmigratory as well as migratory subsets of precursor cells; it detects prethymic and intrathymic precursor cells with equal facility; and it produces a discrete, self-limited wave of donor-origin thymocytes and peripheral T cells. Moreover, neither hemopoietic nor lymphopoietic stem cell chimerism occurs at extrathymic sites. Comparison of the kinetics of thymocytopoiesis in the i.t. and i.v. transfer systems suggest that the seeding efficiency of prothymocytes in the i.v. assay approximates 0.04; the lag phase of the time-response curve is not due to a delay in the entry of prothymocytes into the thymus; and the relative amount of thymocyte precursor activity in various lymphohemopoietic tissues is highest in bone marrow, lowest (or absent) in lymph node, and intermediate in spleen, blood, and thymus. Moreover, the occurrence of saturation kinetics in the dose-response curve of the i.t. system supports the hypothesis that a finite number of microenvironmental niches for prothymocytes may exist in the thymus. These initial observations will require confirmation and extension in future studies. However, based on the present findings and related observations, we anticipate that the i.t. adoptive transfer system will contribute importantly to the definitive analysis of both normal and abnormal thymocytopoiesis.

Analysis of rat hemopoietic cells on the fluorescence-activated cell sorter. I. Isolation of pluripotent hemopoietic stem cells and granulocyte-macrophage progenitor cells.

A scheme is presented whereby pluripotent hemopoietic stem cells (PHSC) from rat bone marrow can be enriched 320-fold with the aid of the fluorescence- activated cell sorter. This scheme is based on the observations that PHSC are strongly positive for Thy-1 antigen (upper 10th percentile); have light- scattering properties (size distribution) between those of bone marrow lymphocytes and myeloid progenitor cells; and are relatively resistant to cortisone. It is estimated that PHSC may constitute 80 percent of the cells isolated according to these parameters. Candidate PHSC are described at the light and electron microscopic levels. At least two populations of accessory cells appear to influence the number and/or the nature of the hemopoietic colonies that form in the in vivo spleen colony-forming unit assay. Putative amplifier cells are strongly Thy-1(+) and cortisone sensitive; putative suppressor cells are weakly Thy-1(+) and cortisone resistant. Three subsets of granulocyte (G) -macrophage (M) progenitor cells (in vitro colony-forming cells [CFC]) are identified on the basis of relative fluorescence intensity for Thy-1 antigen: G-CFC are strongly Thy-l(+); M-CFC are weakly Thy-l(+); and cells that produce mixed G and M CFC have intermediate levels of Thy-1. GM-cluster-forming cells and mature G and M are Thy-1(-). The results suggest that G-CFC are bipotential cells that give rise to G and M-CFC; and that the latter produce mature M through a cluster- forming cell intermediate. Thy-1 antigen is also demonstrated on members of the eosinophil, megakaryocyte, erythrocyte, and lymphocyte cell series in rat bone marrow. In each instance, the relative concentration of Thy-1 antigen is inversely related to the state of cellular differentiation.

Demonstration of Thy-1 antigen on pluripotent hemopoietic stem cells in the rat.

Three approaches were used to demonstrate the presence of Thy-1 antigen on the surface of pluripotent hemopoietic stem cells in the rat. In the first, stem cells from fetal liver, neonatal spleen, and adult bone marrow were prevented from forming hemopoietic colonies in the spleens of irradiated recipients spleen (colony-forming unit assay) by incubation with antibodies to Thy-1 antigen. Highly specific rabbit heteroantiserum to purified rat brain Thy-1 antigen and mouse alloantisera to Thy-1.1-positive thymocytes were equally effective. This inhibition was neutralized by purified Thy-1 antigen. In a second series of experiments, Thy-1-positive and Thy-1-negative populations of nucleated bone marrow cells were separated by the FACS. All of the hemopoietic stem cell activity was recovered in the Thy-1-positive population. The stem cells were among the most strongly positive for Thy-1 antigen, being in the upper 25th percentile for relative fluorescence intensity. The relationships of Thy-1 antigen to the rat bone marrow lymphocyte antigen (BMLA) was shown in a third series of experiments. Rabbit anti-BMLA serum, which is raised against a null population of lymphocyte-like bone marrow cells, has been shown to have anti-stem cell activity. Here we demonstrate by double immunofluorescence, cocapping, and differential absorption studies that Thy-1 and BMLA are parts of the same molecule.

A selective culture system for generating terminal deoxynucleotidyl transferase-positive (TdT+) lymphoid precursor cells in vitro. I. Description of the culture system.

A primary xenogeneic culture system has been devised that selectively generates undifferentiated TdT+ lymphoblasts from rat bone marrow under conditions that do not support the growth or maintenance of rat colony-forming unit-spleen (CFU-S) or granulocyte/macrophage colony-forming cells (GM-CFC). The culture system requires a mouse bone marrow feeder layer, and a serum supplement that has markedly reduced levels of cortisol. The growth of TdT+ cells can be significantly enhanced by the addition of mesodermalizing factors (e.g., fibroblast growth factor, guinea pig bone marrow extract) to the culture medium, and the serum supplement can be decreased by the addition of selenium, transferrin, and T3. The cultured TdT+ cells are antigenically "null" cells that further resemble their normal counterparts in bone marrow with respect to morphology, size, cortisone sensitivity, and pattern of TdT fluorescence. The TdT+ cells are generated with equal facility from bone marrow of normal and congenitally athymic rats, can be maintained in logarithmic growth for at least 10 mos by serial passage in vitro, and do not cause leukemia when infused into irradiated recipients. Although the lineage relationships of these immature lymphoid cells have not yet been established, our working hypothesis, based on preliminary evidence, is that the cultured TdT+ cells are primitive members of the T cell series.

Identification of thymocyte progenitors in hemopoietic tissues of the rat. II. Enrichment of functional prothymocytes on the fluorescence-activated cell sorter.

A quantitative thymocyte regeneration assay was used to monitor the isolation of functional prothymocytes from rat bone marrow on the FACS. Two prothymocyte subpopulations were tentatively identified on the basis of their relative resistance to dexamethasone. Both populations were comprised of undifferentiated, medium-size cells that displayed large amounts of Thy-1 antigen. Simultaneous sorting of bone marrow cells according to relative low angle light scatter (size) and relative fluorescence intensity for Thy-1 resulted in enrichments of 112-fold and 260-fold, respectively, in prothymocyte activity in untreated and dexamethasone-treated bone marrow. These prothymocyte-enriched cell fractions contained or approximately 75% of total functional prothymocyte activity in bone marrow, and represented 1.1 and 0.35% of total untreated and dexamethasone-treated bone marrow cells. Using these enriched cell fractions, significant thymocyte regeneration is possible with as few as 2 X 10(4) and 1 X 10(4) bone marrow cells, respectively. The possible relationship of these functional prothymocyte subpopulations with CFU-S and with TdT-positive cells is discussed.

Human immunity to the meningococcus. I. The role of humoral antibodies.

Susceptibility to systemic meningococcal disease is related to a selective deficiency of humoral antibodies to pathogenic strains of meningococci. In a study of the age-specific incidence of meningococcal meningitis in the United States, it was found that the proportion of individuals with serum bactericidal activity to meningococci of serogroups A, B, and C was reciprocally related to the incidence of disease. The prevalence of bactericidal activity was highest at birth and among adults, and lowest in infants between 6 and 24 months of age. Sera from 51 of 54 prospective cases of meningococcal disease among military recruits were deficient in antibodies to homologous and heterologous strains of pathogenic meningococci as determined by serum bactericidal activity and indirect immunofluorescence. Such sera, however, could support the bactericidal activity of purified human gamma globulin (Cohn fraction II), and such individuals could respond immunologically to infection with meningococci. The implication is that susceptible persons are deficient in antimeningococcal antibodies because they have not received significant exposure to meningococcal antigens in the past. The fate of individuals who lack bactericidal antibodies to pathogenic meningococci was determined during an outbreak of group C meningitis among military recruits. The incidence of disease was found to be primarily associated with the incidence of exposure of susceptibles to the pathogenic strains. Whereas 81.5% of the presumed susceptibles acquired a meningococcal strain, only 24.1% acquired an organism similar to the prevalent disease-producing strains. Of the exposed susceptibles, 38.5% developed systemic meningococcal disease.

Human immunity to the meningococcus. IV. Immunogenicity of group A and group C meningococcal polysaccharides in human volunteers.

High molecular weight group A and group C meningococcal polysaccharides had no significant toxicity for mice or guinea pigs. Furthermore, these polysaccharide preparations contained negligible amounts of biologically active endotoxin. The high molecular weight group A and group C meningococcal polysaccharides were excellent immunogens in six human volunteers. Antibodies belonging to the immunoglobulin classes IgG, IgM, and IgA were produced. These antibodies were highly meningococcocidal in the presence of complement.

Human immunity to the meningococcus. V. The effect of immunization with meningococcal group C polysaccharide on the carrier state.

Purified meningococcal polysaccharides were administered to army recruits. No adverse reactions were observed in 145 men who received group C polysaccharide, and in 53 men who were injected with group A polysaccharide. Hemagglutinating and bactericidal activity developed in the sera of all individuals with the exception of two recruits injected with A polysaccharide. During the 6 wk period of observation, the proportion of unvaccinated recruits who acquired group C meningococci in the three companies studied was 38, 42, and 69 per cent. A significantly lower proportion of the individuals vaccinated with group C polysaccharide acquired group C meningococci; 4.6, 24, and 31 per cent respectively.

Human immunity to the meningococcus. II. Development of natural immunity.

Results of the present study suggest that natural immunity to meningococcal disease is initiated, reinforced, and broadened by intermittent carriage of different strains of meningococci throughout life. In young adults, carriage of meningococci in the nasopharynx is an efficient process of immune sensitization. 92% of carriers of serogroup B, C, or Bo meningococci were found to develop increased titers of serum bactericidal activity to their own meningococcal isolate, and 87% developed bactericidal activity to heterologous strains of pathogenic meningococci. The rise in bactericidal titer occurred within 2 wk of onset of the carrier state, and was accompanied by an increase in titer of specific IgG, IgM, and IgA antibodies to meningococci. In early childhood, when few children have antibodies to pathogenic meningococci, active immunization seems to occur as a result of carriage of atypical, nonpathogenic strains. Immunity to systemic meningococcal infection among infants in the neonatal period is associated with the passive transfer of IgG antibodies from mother to fetus. The antigenic determinants which initiate the immune response to meningococci include the group-specific C polysaccharide, cross-reactive antigens, and type-specific antigens.

Defective lymphopoiesis in the bone marrow of motheaten (me/me) and viable motheaten (mev/mev) mutant mice. III. Normal mouse bone marrow cells enable mev/mev prothymocytes to generate thymocytes after intravenous transfer.

Bone marrow prothymocytes from me/me and mev/mev mutant mice fail to generate thymocytes in irradiated (600 rad) +/+ wild-type recipients after intravenous injection. However, these same prothymocytes readily generate thymocytes after intrathymic injection. The results of the present study demonstrate that this apparent defect in the thymus-homing capacity of mev/mev prothymocytes can be corrected by mixing irradiated wild-type bone marrow cells with mev/mev bone marrow cells before intravenous injection. However, this defect is not corrected by passage of mev/mev bone marrow cells through the bone marrow of irradiated wild-type recipients. One interpretation of these results is that the maturation of prothymocytes is reversibly arrested in mev/mev mice by a defect in the radiosensitive compartment of the bone marrow microenvironment.

Analysis of rat hemopoietic cells on the fluorescence-activated cell sorter. II. Isolation of terminal deoxynucleotidyl transferase-positive cells.

A method is described by which highly enriched populations of viable terminal deoxynucleotidyl transferase-positive (TdT+) cells can be isolated from rat bone marrow by use of the fluorescence-activated cell sorter. Such cells have been postulated to be progenitors of thymocytes and, possibly, of B lymphocytes, and may serve as the targets of neoplastic transformation in acute lymphoblastic leukemia. The separation procedure is based on differences in relative low-angle light scatter and relative fluorescence intensity for Thy-1 antigen between TdT+ cells and other lymphohemopoietic cell populations in bone marrow. Simultaneous sorting of bone marrow cells according to these two parameters resulted in a mean 87% purification of TdT+ cells. The morphological characteristics of the isolated TdT+ cells are described at the light and electron miscroscopic levels.

Defective lymphopoiesis in bone marrow of motheaten (me/me) and viable motheaten (mev/mev) mutant mice. I. Analysis of development of prothymocytes, early B lineage cells, and terminal deoxynucleotidyl transferase-positive cells.

This study identifies defects in the early stages of lymphopoiesis that may contribute to the abnormalities in the development and/or function of peripheral T and B lymphocytes in mice homozygous for the motheaten (me/me) and viable motheaten (mev/mev) mutations. The results indicate that in me/me and mev/mev mice prothymocytes in bone marrow are present in essentially normal numbers, as determined by intrathymic injection, but apparently lack the ability to home effectively to the thymus, as determined by intravenous transfer; early B lineage cells in bone marrow, identified by the B220 antigen, are markedly depleted, including immature B cells (sIg+), pre-B cells (cIg+, sIg-), and pro-B cells (B220+, cIg-, sIg-); TdT+ bone marrow cells, especially a subset that expresses the B220 B lineage antigen, are markedly depleted by two weeks of age; normal numbers of TdT+ thymocytes are present during the first 3 wk of postnatal life, but rapidly decrease thereafter. The results further indicate that neither the defective thymus homing capacity of prothymocytes nor the deficiency of TdT+ bone marrow cells is due to autoantibodies. The possible relationship of the defective development of lymphoid precursor cells to the premature onset of thymic involution and to the abnormalities of peripheral T and B lymphocytes in me/me and mev/mev mice is discussed; as are the results of in vitro studies (presented in a companion paper), which suggest that a primary defect in the stromal microenvironment of the bone marrow is responsible for the abnormal development of the lymphoid precursor cells.

Tissue-specific cell-surface antigens in embryonic cells.

With the use of antisera prepared in rabbits against suspensions of live embryonic chick tissue cells, qualitative differences in cell surface antigens were demonstrated on cells from different embryonic chick tissues by immune agglutination and immunofluorescence. Unabsorbed antisera reacted with both homologous and nonhomologous cells; thorough absorption of the antisera with heterologous tissues removed cross-reacting antibodies, and the antisera acquired a high degree of tissue specificity. Thus, antiretina cell serum absorbed with nonretina cells or tissues, agglutinated only neural retina cells, and was shown by immunofluorescence tests to react specifically with the surface of retina cells, both in cell suspensions and in frozen tissue sections. Comparable results with antisera against cells from embryonic liver and other tissues demonstrated the existence of tissue-specific, phenotypic disparities in the antigenicities of embryonic cell surfaces, in addition to the presence of cell-surface antigens shared by certain classes of cells, and of antigens common to all cells in the embryo. The results are discussed in terms of the possible involvement of such phenotypic determinants in the specification of cell surfaces, in relation to cell recognition and developmental interactions.

Clinical evaluation of group A and group C meningococcal polysaccharide vaccines in infants.

Group A and group C meningoccal polysaccharide vaccines were evaluated in infants. No significant local or systemic reactions were observed with 908 doses of vaccine given to 396 infants between 3 and 12 mo of age. The antibody response varied with the age of the infant, vaccine dose, molecular weight of vaccine, prior immunization with vaccine, and prior exposure to naturally occurring cross-reactive antigens. Only 7% of 3-mo-old infants had detectable antibody responses to primary immunization with 5-200 mug of A vaccine, presumably because of suppressive effects of high concentrations of maternal anti-A. More than 90% of 7- and 12-mo-old infants responded to A vaccine, achieving geometric mean anti-A concentrations of 0.38 and 0.98 mug/ml, respectively. The dose-response curve was flat between 10 and 200 mug of A vaccine. Geometric mean anti-A concentrations of 2.51 and 4.00 mug/ml were induced in 7- and 12-mo-old infants by booster injections of A vaccine. Approximately 90% of 3-mo-old infants had detectable antibody responses to primary immunization with C vaccine. The 100-mug dose appeared to be optimal, resulting in geometric mean anti-C concentrations of 0.49, 1.55, and 2.64 mug/ml in 3-, 7-, and 12-mo-old infants, respectively. Significant booster responses were not observed with C vaccine. Indeed, except for the 10-mug dose, booster injections of C vaccine in 7- and 12-mo-old infants resulted in lower anti-C concentrations than did primary immunizations.