A comparison of changes in the stereocilia between temporary and permanent hearing losses in acoustic trauma

A comparison of stereociliary changes at different post-exposure intervals in ears with temporary and permanent hearing losses has been made. Twenty guinea pigs were exposed to either 110 dB SPL broadband white noise for 30 min (N = 10) or 120 dB SPL white noise for 150 min (N = 10). The recovery patterns for threshold shifts for both groups were systematically assessed at regular post-exposure intervals for 80 days, using the auditory cortex evoked response to tone bursts between 0.5 and 8kHz. Thirty-two animals that had been exposed to the same noise at either 110 dB for 30 min (N = 16) or 120 dB for 150 min (N = 16) were decapitated for scanning electron microscopic examination at the same post-exposure intervals. The threshold shifts induced by 110 dB noise were reversible while those induced by 120 dB were generally irreversible, although extreme variabilities existed among the animals. In the acute TTS ears, damage was confined to the third row of OHCs, where only the tips of the stereocilia were affected. Neither discontinuity of cuticular plate nor expelled cytoplasm was found in these cells. In the lesions of PTS, either all the three rows of OHCs or the IHCs and the first row of OHCs were involved. The entire length of the stereocilia, more severe in the lower part was always damaged. Expelled cytoplasm and fusion between stereocilia were frequently seen. In the chronic TTS ears, no abnormalities of stereocilia were found while in the PTS ears, a complete absence of the organ of Corti was noticed. The results of the present study clearly suggest that the status of the lower part of the stereocilia and the continuity of the cuticular plate play an important role in determining the reversibility of threshold shifts.

Evidence that the apparent complexity of receptor antagonism by angiotensin II analogues is due to a reversible and syntopic action

1. The interactions between angiotensin II (AII), two non-peptide antagonists DuP 753 and IMI, and eight peptide analogues of AII were investigated on the rabbit isolated aorta assay. DuP 753 and IMI behaved as simple competitive antagonists (pKB values 8.4 and 6.8, respectively). To different degrees, all the AII-peptide analogue interactions failed to meet the basic criteria for simple competition. In addition to rightward shift, the most significant feature was a concentration-dependent saturable depression of the upper asymptote of the AII concentration-effect curves. 2. 'Washout' and combined dose-ratio analysis experiments, in which DuP 753 was used as a reference antagonist, indicated that the profile of peptide antagonism was solely due to a reversible and syntopic action at the AII receptor. 3. By use of an operational model of agonism (Black & Leff, 1983) as a starting point, it was possible to account for the data with a new model which describes reversible receptor occupancy and occupied receptor-determined, saturable reduction in the efficacy of AII. Model-fitting gave estimates of pKB values for the peptide analogues and agonist affinity and efficacy parameters for AII. 4. The model was successfully tested by applying it to qualitatively similar results obtained in a cross-tissue analysis on guinea-pig aorta, ileum and stomach. 5. A 'molecular' interpretation of the efficacy changes, based on the concepts of receptor internalisation and expression, is offered.

Maturation and dispersal of B-cell clones during T cell-dependent antibody responses

Germinal centers develop in follicles during TD antibody responses in the first 3 wk following each immunization. In primary responses to protein-based antigens, T-cell help is limiting, follicles develop towards the end of the 1st wk from immunization and the size of the follicular response in relatively small. When T-cell help is provided the primary B-cell response in follicles is much larger, B cells start to proliferate in follicles within a few hours of immunization and reach peak size 3-4 d later. Available evidence suggests that virgin B cells that colonize follicles to form germinal centers must first be activated by antigen outside follicles, probably in T zones. Memory B cells also proliferate in follicles and they can do so without first being activated outside follicles. Germinal center formation consists of an initial phase of exponential proliferation of B cells within the follicular dendritic cell network. After a single immunization the follicular response is oligoclonal and on average only 3 cells colonize each follicle. In responses to hapten-protein in rats primed previously with the carrier protein these 3 cells increase to around 10(4) cells in 3 d with a cell cycle time of about 6 h. At the end of the period of exponential growth of B blasts, the classical structure of germinal centers emerges. The B blasts become centroblasts in the dark zone of the germinal center which develops at that pole of the FDC network nearer the T zones. The centroblasts are still in rapid cell cycle but do not more than sustain their numbers. The rest of their progeny move to the heart of the FDC network where they come out of cell cycle as centrocytes. Evidence is cited which indicates that somatic mutation occurs in the Ig V-region genes of centroblasts and that centrocytes are selected on the basis of their ability to respond to antigen held on FDC. Centrocytes not receiving this antigen-dependent signal kill themselves by apoptosis. Centrocytes positively selected by interaction with antigen on FDC receive further signals which induce the cells to differentiate to become either plasma cells or memory B cells. The nature of some of these differentiation signals is described. It is shown that proliferation, selection and differentiation occur within germinal centers in distinct micro-environments.(ABSTRACT TRUNCATED AT 400 WORDS)

Germinal centres in T-cell-dependent antibody responses

For more than a century follicles have been recognized as a site of intense cell proliferation and cell death. At last the significance of this activity is beginning to emerge: antigen-driven B-cell proliferation, somatic mutation, positive and negative selection, and memory and plasma cell development all appear to take place within the follicle.

Growth factor requirements for the stimulation of germinal center B cells: evidence for an IL-2-dependent pathway of development

Germinal center (GC) B cells readily undergo apoptosis, a tendency which can be suppressed in vitro by immobilized anti-Ig; mAb to CD40 and soluble CD23 (in synergy with IL-1 alpha) also effect rescue of GC cells from programmed cell death. In the present study, the signals which stimulate rescued GC populations to DNA synthesis have been examined and compared to those established for the activation of follicular mantle (FM) B cells. On co-culture with anti-Ig, optimal responses in FM B cells can be achieved with a combination of IL-4 and CD40 antibody; these activities also provided a modest stimulus to GC cells but, for this population, anti-Ig was ineffective at augmenting the response further. Stimulations of GC B cells were enhanced, however, when performed on a support of primary fetal lung fibroblasts; a major influence of stroma was to promote, by direct cell-cell contact, the CD40-dependent survival of GC B cells. FM B cells were relatively independent of such stromal support. In marked contrast to FM cells, GC B cells were found to respond by enhanced DNA synthesis to IL-2 even when quite low concentrations of the factor were present (IC50 = 2 U/ml). Stimulation of GC cells via this pathway was augmented almost 2-fold on the inclusion of anti-Ig whereas neither fibroblasts, IL-4, nor CD40 antibody made any additional contribution to the IL-2-dependent response. The requirements found for stimulating GC cells in vitro are discussed with reference to the signals that this population may encounter in appropriate microenvironments in vivo: the variety of options apparently available could reflect changing priorities at different stages of a developing GC response.

Sites of specific B cell activation in primary and secondary responses to T cell-dependent and T cell-independent antigens

Techniques which identify hapten-specific B cells in tissues have been used to determine the sites of B cell activation in rat spleens in response to T cell-dependent (TD) antigens and T cell-independent type-1 (TI-1) antigens. Surface-associated hapten binding by specific memory B cells and B blasts was distinguished from the strong cytoplasmic hapten binding by specific plasma cells and plasmablasts. Blast cells in S phase were identified in tissue sections by staining cells which had been pulse labeled in vivo with 5-bromo-2'-deoxyuridine. Hapten-specific B blast cells are found in three sites: (a) around interdigitating cells in the T cell-rich zones; (b) in the follicular dendritic cell network and (c) in association with macrophages in the red pulp. Hapten-binding memory B cells, which are not in cell cycle, accumulate in the marginal zones and to a lesser extent the follicular mantles in response to TD and TI-1 antigens. The hapten-specific blast response in T zones is confined to the first few days after antigen is given and is low for primary responses to TD antigens, but massive on secondary challenge, when marginal zone memory B cells migrate to the T zones. Both the primary and secondary T zone responses to TI-1 antigens are impressive and in these responses hapten-specific B blasts are also found in the splenic red pulp. The follicular response to TD antigens starts with a small number of B blasts (fewer than five) entering each follicle. These increase in number exponentially so that by the 4th day after immunization they fill the follicle. The oligoclonality of the response is shown in simultaneous responses to two haptens where 6%-31% of the follicles on day 3 after immunization contain blasts specific for only one of the two haptens. During the 4th day classical zonal pattern of germinal centers develops. The surface immunoglobulin-positive B blasts are lost from the follicle center, while one pole of the follicular dendritic cell network fills with surface immunoglobulin-negative centroblasts. Centroblasts do not increase in numbers but divide to give rise to centrocytes, which re-express sIg and migrate into the follicular dendritic cell network. Cell kinetic studies indicate that the centrocyte population is renewed from centroblasts every 7 h. Centrocytes either leave the germinal center within this time or die in situ.(ABSTRACT TRUNCATED AT 400 WORDS)

Germinal center cells express bcl-2 protein after activation by signals which prevent their entry into apoptosis

B cells undergo selection within germinal centers on the basis of their capacity to be activated by antigen held on follicular dendritic cells. Isolated germinal center B cells in culture kill themselves by apoptosis but this is prevented if their receptors for antigen are cross-linked. In this study it is confirmed that almost all germinal center B cells, unlike other B cells, do not express the 25-kDa protein encoded by the bcl-2 oncogene. Cross-linking the surface Ig of isolated germinal center cells causes them to express bcl-2 protein. Two other stimuli which inhibit the entry of germinal center cells to apoptosis result in the expression of bcl-2 protein. These stimuli are: (a) CD40 antibody and (b) recombinant 25-kDa fragment of the CD23 protein plus recombinant interleukin 1 alpha. Respectively, these induce germinal center cells to differentiate to resting B cells or plasmablasts. Dual-fluorescence studies on small lymphocytes confirm the presence of bcl-2 protein in mitochondria but show that this is also present in other extra-nuclear areas. Burkitt lymphoma cells have a phenotype which indicates that they are neoplastic cells of germinal center origin. The expression of bcl-2 protein by Burkitt lymphoma lines was also studied. Burkitt lines which retain the phenotype of fresh Burkitt lymphoma cells can be induced to enter apoptosis on culture with the Ca2+ ionophore ionomycin. These cells were found not to express bcl-2 protein. By contrast, Burkitt lines which have drifted towards a lymphoblastoid cell line phenotype and are resistant to the induction of apoptosis express high levels of the bcl-2 protein. The findings support the concept that the susceptibility of germinal center cells to entering apoptosis is associated with their lack of expression of bcl-2 protein. Aberrant expression of bcl-2 protein by some neoplastic germinal center cells may allow survival in situations where their normal counterparts die.

Recombinant 25-kDa CD23 and interleukin 1 alpha promote the survival of germinal center B cells: evidence for bifurcation in the development of centrocytes rescued from apoptosis

Germinal centers contain a proliferating pool of centroblasts which give rise to non-dividing centrocyte. Centrocytes are programmed to die by apoptosis unless they receive a positive signal for rescue. Rescue, in vivo, is likely to be dependent, initially, on interaction with antigen held on follicular dendritic cells (FDC). A subset of FDC located in that part of the germinal center furthest from centroblasts is particularly rich in CD23. Supernatants containing high levels of soluble CD23 were found not only to encourage the survival of germinal center B cells but also to promote their differentiation toward a plasmacytoid morphology; these activities were diminished following removal of CD23 from the supernatants. Recombinant 25-kDa CD23 was initially found to be incapable of providing the signal for germinal center cell development but on the addition of interleukin 1 alpha which, by itself, was inactive, rescue and differentiation of germinal center B cells were now achieved. Apoptosis in germinal center cells could also be prevented by the ligation of surface CD40 with monoclonal antibody: however, rescue via this pathway was not accompanied by plasmacytoid differentiation. These findings provide a functional rationale to the high level expression of CD23 found within a discrete subset of FDC and indicate a bifurcation in the development of germinal center B cells following their rescue from apoptosis.

[Amniotic band disruption complex: report of one case in twins]

Amniotic band disruption complex is a congenital disorder with the principal features vary from simple digital band constrictions to major craniofacial and visceral defects. Early amniotic rupture results in multiply affected infants who are frequently aborted or stillborn, whereas later rupture results primarily in limb involvement. Here we report a case of amniotic band disruption complex, a male infant (twin A) weighing 2350 gm with Apgar scores of 9 and 10 at one and five minutes. The second, third and fourth fingers on the left hand appeared to have ring constrictions, the distal part of the fourth finger had been amputated distally. There were amputations and ring constrictions of the distal parts of the second, third and fifth digits on the right hand, the fourth finger had only proximal third. All of the toes on the left foot had been amputated distally. The fifth toe on the right foot were absent, and the others had been partly amputated. Left cleft lip and palate were present with sunken left nasal flare. This baby underwent the cranial ultrasound examination, which demonstrated on structural anomalies, and both of the twins had the karyotype of 46XY. No other abnormalities was noted in the infant. The 890 gm placenta was delivered spontaneously. Two umbilical cords were present, each with normal vessels. The examination of the placenta revealed it to be dichorionic and diamniotic, and numerous fibrous strings extending form the fetal surface.

[Dandy-Walker complex with multiple anomalies: report of one case]

A 3570 gram male newborn was born to a 29-year-old mother at the gestational age of 40 weeks with gross abnormalities of flexion contracture of third and fifth fingers and fourth toes bilaterally. After birth, cyanotic spell while feeding was found, and a series of examinations were done. The abnormalities were focused on brain echogram, computed tomogram and magnetic resonance imaging examination, which showed a large posterior fossa cyst, cerebellar hypoplasia complicated with multiple severe supratentorial anomalies, including marked ventricular dilatation, cerebral agyria, agenesis of corpus callosum, absence of thalamus and basal ganglion, and optic nerve atrophy. Clinically, the patient had neonatal seizure and high fever. The diagnosis of Dandy-Walker complex type A was made according to the new classification presented by Barkovick et al. in 1989.

Compound I radical in site-directed mutants of cytochrome c peroxidase as probed by electron paramagnetic resonance and electron-nuclear double resonance

The reaction of ferric cytochrome c peroxidase (CcP) from Saccharomyces cerevisiae with peroxide produces compound I, characterized by both an oxyferryl iron center and a protein-based free radical. The electron paramagnetic resonance (EPR) signal of the CcP compound I radical can be resolved into a broad majority component which accounts for approximately 90% of the spin intensity and a narrow minority component which accounts for approximately 10% of the integrated spin intensity [Hori, H., & Yonetani, T. (1985) J. Biol. Chem. 260, 3549-3555]. It was shown previously that the broad component of the compound I radical signal is eliminated by mutation of Trp-191 to Phe [Scholes, C. P., Liu, Y., Fishel, L. F., Farnum, M. F., Mauro, J. M., & Kraut, J. (1989) Isr. J. Chem. 29, 85-92]. The present work probed the effect of mutations in the vicinity of this residue by EPR and electron-nuclear double resonance (ENDOR). These mutations were obtained from a plasmid-encoded form of S. cerevisiae expressed in Escherichia coli [Fishel, L. A., Villafranca, J. E., Mauro, J. M., & Kraut, J. (1987) Biochemistry 26, 351-360]. The EPR line shape and ENDOR signals of the compound I radical were perturbed only by mutations that alter Trp-191 or residues in its immediate vicinity: namely, Met-230 and Met-231, which have sulfur atoms within 4 A of the indole ring, and Asp-235, which forms a hydrogen bond with the indole nitrogen of Trp-191. Mutations of other potential oxidizable sites (tryptophan, tyrosine, methionine, and cysteine) did not alter the EPR line shapes of the compound I radical, although the integrated spin intensities were weaker in some of these mutants. Mutations at Met-230 and/or -231 perturbed the EPR line shapes of the compound I radical signal but did not eliminate it. ENDOR of these two methionine mutants showed alteration to the hyperfine couplings of several strongly coupled protons, which are characteristic of the majority compound I radical electronic structure, and a change in weaker hyperfine couplings, which suggests a different orientation of the radical with respect to its surroundings in the presence of these methionine mutations. Besides the Trp-191----Phe mutation, only the Asp-235----Asn mutation eliminated the broad component of the compound I signal. Loss of the broad compound I EPR signal coincides with both the loss of the Asp----Trp-191 hydrogen-bonding interaction and alteration of the position of the indole ring of Trp-191.(ABSTRACT TRUNCATED AT 400 WORDS)

The biology of mesangial cells in glomerulonephritis

It is likely that a complex bidirectional interaction occurs between mesangial cells and the immune cells which infiltrate the mesangium during nephritis. Macrophages and other immune cells liberate a series of mediators, including substances such as IL-1, beta-endorphin, TNF, and PDGF--all of which promote the growth of mesangial cells. The end result is mesangial cell proliferation and increased matrix production, both of which are seen in nephritis. The proliferating mesangial cells liberate autocoids such as IL-1 and PDGF, thereby setting up an amplifying loop. Simultaneously, suppressive factors such as TGF-beta are released which antagonize the actions of these growth-promoting substances. The proliferating mesangial cells also produce immunomodulatory peptides, which will in turn act on the infiltrating macrophages to stimulate their replication and activation. Such activated macrophages continue to amplify the inflammatory lesion and also promote the phagocytosis of localized antigen-antibody complexes. The net effect of all of these interactions will depend on the dominance of substances which persist and override the roles of other molecules. Studies of the controls which regulate the production of these growth factors/immune modulators will yield insights into the fundamental mechanisms which determine the outcome in glomerulonephritis.

[Hemolytic disease of the newborn caused by maternal irregular antibody anti-E: report of one case]

A 3390 gm male baby was born smoothly at 41 weeks' gestation to a 35-year-old, gravida 3, para 3, mother by spontaneous vaginal delivery at this hospital. The mother denied ever having received a blood transfusion. Her first two uneventful pregnancies had resulted in two healthy fullterm infants. Physical examination of the baby showed normal, until 15 hours of age, when apparent jaundice with serum total bilirubin concentration up to 21 mg/dl was noted. The blood types of both baby and mother were group A, Rh(D) positive. Compatibility testing for exchange blood transfusion with group A whole blood showed positive reaction due to the presence of an irregular antibody. Two exchange blood transfusions were performed in the same day with compatible blood. Thereafter, the patient's condition was much improved, he was discharged one week later. The irregular antibody--anti-E was identified in both baby and mother. The maternal RBC phenotyping was CCDee (R1R1), and the baby, CcDEe (R1R2).

Electron nuclear double resonance (ENDOR) of the Qc.- ubisemiquinone radical in the mitochondrial electron transport chain

We present an electron nuclear double resonance (ENDOR) study of the bound Qc.- ubisemiquinone in the mitochondrial quinol cytochrome c reductase complex. An ENDOR probe specifically modified for insertion into our electron paramagnetic resonance cavity was used for this study. We observed strongly hyperfine-coupled protons whose exchangeable nature indicated they were hydrogen-bonded to the quinone oxygen(s). It is thought that such hydrogen bonds are critical in binding the ubiquinone to protein, in stabilizing its semiquinone form, and in modulating the thermodynamic properties of the bound ubiquinone in the mitochondrial quinol cytochrome c reductase complex. Additional ENDOR features were assigned to protons of the quinone ring itself and to weakly coupled protons that may be associated with nearby amino acids. From very weakly hyperfine-coupled, distant, exchangeable protons there was also ENDOR evidence to suggest proximity and accessibility of the ubiquinone site to the solvent.

Deep cerebral infarcts in the People's Republic of China

We reviewed computed tomograms and clinical characteristics for 1,124 consecutive stroke patients from Shanghai, People's Republic of China. While there were many similarities between lacunar syndromes in these patients and patients from Europe and the United States, several differences were noted. Deep cerebral infarcts identified by computed tomography were more common in patients from Shanghai and accounted for 27% of all ischemic infarcts. Small deep infarcts occurred more commonly in women from Shanghai (44%) than in women from the West. Deep cerebral infarcts in patients from Shanghai were larger than those usually seen in patients from the West. The mean infarct volume in patients from Shanghai was 2.4 ml, and 49% were giant lacunes (greater than 15 mm in maximal diameter).

The evolution of B-cell clones

This chapter identifies three forms of B-cell memory: (a) B blasts which characterize the established stage of the follicular response to TD antigens, (b) recirculating memory B cells, and (c) non-recirculating memory B cells of the marginal zones of the spleen and equivalent areas of other secondary lymphoid organs. The follicular B blasts show sustained proliferation driven by small amounts of antigen bound to FDCs. The probable relationships between these cells is summarized diagrammatically in Fig. 4. It is probable that follicular B blasts generate both the recirculating and marginal zone memory cells. The chapter by Gray and Leanderson in this volume cites data which indicate that the recirculating memory pool is not sustained for more than a few weeks in the absence of antigen. Data leading to the same conclusion for marginal zone memory B cells is set out in Sect. 5.1 of this chapter. Marginal zone memory B cells do not appear to move spontaneously to follicles for periodic renewal. They will only leave the marginal zone if a fresh supply of antigen reaches them in that site. Recirculating B cells are able to respond to antigen already held on FDCs. It is not known if they are able to displace B blasts of equivalent affinity for antigen which already occupy antigen-holding sites on FDCs. This could be a mechanism by which B blasts with high antigen affinity produced in one follicle could displace blasts of lower affinity in other follicles. Little is known of the factors which regulate the numbers of marginal zone and recirculating follicular memory B cells. In responses to hapten-protein conjugates, hapten-binding cells may approach 10% of marginal zone B cells but comprise well under 1% of recirculating follicular cells. The numbers of these memory cells do not increase if the recirculating pool of lymphocytes is depleted, indicating that the factors which regulate the number of memory B cells are independent of those which regulate the total size of the recirculating B-cell pool. A depleted peripheral B-cell pool can only be fully reconstituted by recruitment of newly produced virgin B cells. Data cited in Sect. 5.2 support the concept that this recruitment is at least partially independent of antigen-driven B-cell proliferation. Consequently, substantial proportions of the peripheral B-cell pools are likely to be either virgin cells or cells which have been recruited by antigen or anti-idiotype without entering cell cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of antigen-driven selection in germinal centres

The high affinity of antibodies produced during responses to T-cell-dependent antigens is associated with somatic mutation in the variable region of the immunoglobulin. Indirect evidence indicates that: (1) this arises by a process of hypermutation, acting selectively on rearranged immunoglobulin variable-region genes, which is activated in centroblasts within germinal centres; and (2) centrocytes, the progeny of centroblasts, undergo selection on the basis of their ability to receive a positive signal from antigen. We have now performed experiments analysing this selection process, and found that, on culture, centrocytes isolated from human tonsil kill themselves within a few hours by apoptosis. This is not a feature of other tonsillar B cells. Centrocytes can be prevented from entering apoptosis if they are activated both through their receptors for antigen and a surface glycoprotein recognized by CD40 antibodies.

B cell memory to thymus-independent antigens type 1 and type 2: the role of lipopolysaccharide in B memory induction

Several studies have indicated that thymus-independent (TI) antigens, unlike their thymus-dependent (TD) counterparts, are poor at generating memory antibody responses (Immunol. Today 1981. 3:217). In contrast to this view, the present report shows that the TI type 1 (TI-1) antigen, 2,4,6-trinitrophenyl-lipopolysaccharide (TNP-LPS), elicits good secondary responses in rats. These secondary antibody responses are not only greater in magnitude than the primary responses, but display a different pattern of Ig classes with more IgG and IgA antibodies produced. In transfer experiments between congenic strains of rats which differ in their kappa light chain Ig allotype, it is shown that this memory is attributable to persistent B cell clones. The TI-2 antigen, 2,4-dinitrophenyl-hydroxyethyl starch (DNP-HES), given alone did not elicit B cell memory. However, when DNP-HES is presented to the immune system in association with LPS, the pattern of the anti-DNP response is similar to that elicited by TNP-LPS. The capacity to generate TI memory is associated with the appearance of hapten-specific B cells in the marginal zones of the spleen. Hapten-binding cells were induced in the marginal zones following immunization with TNP-LPS, but not by DNP-HES. However, concurrent immunization with DNP-HES and LPS which were not covalently linked was found to induce DNP-binding cells in the marginal zone. There is complete correlation between the appearance of hapten-binding memory B cells in the marginal zone and the capacity of these antigens to induce secondary responses.

Is rapid proliferation in B centroblasts linked to somatic mutation in memory B cell clones?

Antigen-reactive B cells accumulate mutations in the variable (v) regions of their immunoglobulin genes during certain phases of T cell-dependent (TD) antibody responses. This is associated with a rise in the affinity of specific antibody. The time when somatic mutations are accumulating seems to coincide with the presence of germinal centres. This has led to the suggestion that a mechanism leading to a high rate of base pair substitution in immunoglobulin v region genes might operate in centroblasts in germinal centres. The rate of accumulation of mutations in v region genes is likely to relate to the number of specific B cells in cycle and their rate of cell division. The present report provides evidence pointing to centroblasts having a remarkably short cell cycle time of some 6 to 7 hours. This rapid rate of proliferation may explain the clonal expansion which occurs in the early phase of TD antibody responses and the efficiency with which high affinity mutants are subsequently selected.

Memory B cells in T cell-dependent antibody responses colonize the splenic marginal zones

Specific hapten-binding B cells were identified in the splenic marginal zones following immunization with hapten-protein conjugates. Hapten binding by marginal zone B cells does not appear to be due to passive absorption of anti-hapten antibody. For double immunization with two haptens, 2,4-dinitrophenyl (DNP) and 2-phenyloxazalone (Ox) each conjugated to hemocyanin, resulting in the appearance of discrete DNP-binding cells and Ox-binding cells in the marginal zone. Very few cells were identified which bound both haptens. The hapten-binding cells in the marginal zones have a phenotype characteristic of other marginal zone B cells. They express surface IgM but not IgD. Occasional cells also have surface IgG2c. All hapten-binding cells possessed the antigen recognized by the monoclonal antibody HIS 14 but lacked those identified by HIS24 and HIS22. Hapten-binding B cells were shown to have been in cell cycle shortly before entering the marginal zone but were no longer in cell cycle after arriving at that site. Once in the marginal zone hapten-binding cells were shown to remain in that site for upwards of 2 weeks. Following reimmunization with DNP-hemocyanin, DNP-binding but not Ox-binding cells were lost from the marginal zone. At the same time DNP-binding cells arrived in the periarteriolar lymphocytic sheath and to a lesser extent the follicles. These cells were in active cycle and appeared to give rise both to plasma cells and marginal zone hapten-binding cells. It is concluded that hapten-binding cells found in the marginal zones are memory B cells i.e. they have been derived from B cells which have undergone antigen-driven proliferation, they are no longer in cell cycle but can be induced to re-enter cell cycle by subsequent exposure to antigen. Good antibody responses were obtained following immunization with hapten-polysaccharides; however, no hapten-binding cells appeared in the marginal zones in response to these T cell-independent type 2 antigens.