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.

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.

Asymptomatic carriage of Neisseria meningitidis and Neisseria lactamica was studied in a total of 2,969 healthy infants and children in Danbury, Conn., between October 1971 and June 1975. The prevalence of N. meningitidis averaged 0.71% during the first four years of life and increased to 5.4% by 14--17 years. Rates of carriage of N. lactamica increased from 3.8% in three-month-old infants to a peak of 21.0% at 18 months and then declined to 1.8% by 14--17 years of age. Of the children who acquired N. lactamica, 66% developed fourfold or greater rises in titers of IgG antibody to groups A, B, and/or C meningococci as determined by immunofluorescence compared with only 5% of control children. Of new carriers of N. lactamica, 40% developed increased titers of bactericidal antibody to groups A, B, and/or C meningococci as compared with 7% of noncarriers. Carriage of N. lactamica may assist in the development of natural immunity to N. meningitidis by induction of cross-reactive antibodies.

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 ∼1 wk; refractory period ∼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.

Many dendritic cells (DCs) in the normal mouse thymus are generated intrathymically from common T cell/DC progenitors. However, our previous work suggested that at least 50% of thymic DCs originate independently of these progenitors. We now formally demonstrate by parabiotic, adoptive transfer, and developmental studies that two of the three major subsets of thymic DCs originate extrathymically and continually migrate to the thymus, where they occupy a finite number of microenvironmental niches. The thymus-homing DCs consisted of immature plasmacytoid DCs (pDCs) and the signal regulatory protein α–positive (Sirpα+) CD11b+ CD8α− subset of conventional DCs (cDCs), both of which could take up and transport circulating antigen to the thymus. The cDCs of intrathymic origin were mostly Sirpα− CD11b− CD8αhi cells. Upon arrival in the thymus, the migrant pDCs enlarged and up-regulated CD11c, major histocompatibility complex II (MHC II), and CD8α, but maintained their plasmacytoid morphology. In contrast, the migrant cDCs proliferated extensively, up-regulated CD11c, MHC II, and CD86, and expressed dendritic processes. The possible functional implications of these findings are discussed.

Several adoptive transfer systems for the detection of thymocyte progenitors have been developed in rats and mice (1-6). Although these assays vary in methodological detail, all use an i.v. route of cell transfer to study the ability of hemopoietic precursors to repopulate the thymus of irradiated recipients. These transfer systems have permitted significant progress in the identification, isolation, and characterization of prothymocytes and in the general analysis of thymocytopoiesis (e.g., 7-10). However, the i.v. transfer systems have several properties that limit their usefulness for detailed studies of thymocyte development. Among these limiting factors are low sensitivity, the requirement for cell migration, and the engraftment of pluripotent hemopoietic stem cells and lymphoid stem cells in extrathymic sites. Consequently, in the i.v. systems, relatively large numbers of precursor cells are required to generate detectable numbers of donor-origin thymocytes; only those subsets of precursor cells that are able to migrate from blood to thymus are detected; and the fine analysis of thymocytopoiesis is obscured by the continual seeding of donor-origin prothymocytes from the chimeric bone marrow to the thymus of the adoptive host. These factors are especially troublesome in the study of intrathymic precursor cells.Herein, we describe a quantitative, intrathymic (i.t.) ~ adoptive transfer assay system that detects the generation of donor-origin thymocytes after the injection of thymocyte precursors directly into the thymus of sublethally irradiated rats and mice. The i.t. transfer assay, being independent of cell migration, is much more sensitive than is the i.v. transfer assay, and can detect subsets of nonmigratory as well as migratory thymocyte precursors. Furthermore, the i.t. transfer assay is entirely selective for T-lineage precursor cells, and readily detects both intrathymic and prethymic populations of precursor cells; yet neither hemoThis work was supported by grant AI-09649 from the National Institute of Allergy and Infectious Diseases, and by a grant from the University of Connecticut Research Foundation. Preliminary reports have been presented [Goldschneider, I., K. Komschlies, and D. L. Greiner. 1985

Integrins contribute to lymphopoiesis, whereas Toll-like receptors (TLRs) facilitate the myeloid replenishment during inflammation. The combined role of TLRs and integrin on hematopoiesis remains unclear. gp96 (grp94, HSP90b1) is an endoplasmic reticulum master chaperone for multiple TLRs. We report herein that gp96 is also essential for expression of 14 hematopoietic system-specific integrins. Genetic deletion of gp96 thus enables us to determine the collective roles of gp96, integrins, and TLRs in hematopoiesis. We found that gp96-null hematopoietic stem cells could support long-term myelopoiesis. B-and T-cell development, however, was severely compromised with transitional block from pro-B to pre-B cells and the inability of thymocytes to develop beyond the CD4 ؊ CD8 ؊ stage. These defects were cell-intrinsic and could be recapitulated on bone marrow stromal cell culture. Furthermore, defective lymphopoiesis correlated strongly with failure of hematopoietic progenitors to form close contact with stromal cell niche and was not the result of the defect in the assembly of antigen receptor or interleukin-7 signaling. These findings define gp96 as the only known molecular chaperone to specifically regulate T-and B-cell development. (Blood. 2010;115: 2380-2390) IntroductionIntegrins are a family of 24 ␣␤ heterodimers in vertebrates formed noncovalently by 18 ␣ and 8 ␤ integrins, of which 17 integrins are expressed in the hematopoietic system. 1,2 Known best for their adhesion properties, integrins also orchestrate signals between extracellular matrix and intracellular cytoskeletons in regulating diverse functions of cells, including proliferation and differentiation. However, despite the expression of integrins on hematopoietic stem cells (HSCs) and the role of integrins in HSC homing to the bone marrow (BM) niche, their function in hematopoiesis remains controversial. For example, although ␣4 integrin has been implicated in both T and B lymphopoiesis from fetal HSCs, 3,4 it appears to play a less significant role in adult hematopoiesis. 5,6 Furthermore, combined deletion of both ␤1 and ␤7 integrins, which are the only known partners of ␣4 integrin, causes no defect in either lymphopoiesis or myelopoiesis. 7 Genetic ␤2 integrin deficiency causes myeloid hyperplasia, including profound granulocytosis and splenomegaly, but no significant problems in hematopoiesis. 8 Clearly, both ␣4 and ␤2 integrins are involved in homing of HSCs in the BM and recruitment of leukocytes to sites of inflammation. 5,9,10 Although pan-integrin deficient system is now available, 11 no resolution of the roles of integrin in hematopoiesis has emerged.Toll-like receptors (TLRs) are pattern recognition receptors that play important roles in sensing pathogen-associated molecular patterns from microbes, which are critical for host immune response. 12 More than 10 TLRs have been described in vertebrates, recognizing a spectrum of microbial moieties, such as endotoxin, flagellin, dsRNA, and DNA. In the steady state, TLRs do not contribute signifi...

A B S T R A C T GroupApproximately 90% of 3-mo-old and 100% of 7-and 12-mlo-old infants had detectable antibody responses to primary immunization with C vaccine. The 100-iug dose appeared to be optimal, resulting in geometric mean anti-C concentrations of 0.49, 1.55, and 2.64 Mug/iln in 3-, 7-, and 12-mo-old infants, respectively. Significalnt booster responses were not observed with C vaccine. Indeed, except for the 10-,ug dose, booster injections of C vaccine in 7-and 12-mo-old ilnfants resulted in lower anti-C concentrations than did primary immunizations.