Role of commensal bacteria in development of gut-associated lymphoid tissues and preimmune antibody repertoire

Intestinal bacteria are required for development of gut-associated lymphoid tissues (GALT), which mediate a variety of host immune functions, such as mucosal immunity and oral tolerance. In rabbits, the intestinal microflora are also required for developing the preimmune Ab repertoire by promoting somatic diversification of Ig genes in B cells that have migrated to GALT. We studied the mechanism of bacteria-induced GALT development. Bacteria were introduced into rabbits in which the appendix had been rendered germfree by microsurgery (we refer to these rabbits as germfree-appendix rabbits). We then identified specific members of the intestinal flora that promote GALT development. The combination of Bacteroides fragilis and Bacillus subtilis consistently promoted GALT development and led to development of the preimmune Ab repertoire, as shown by an increase in somatic diversification of VDJ-C micro genes in appendix B cells. Neither species alone consistently induced GALT development, nor did Clostridium subterminale, Escherichia coli, or Staphylococcus epidermidis. B. fragilis, which by itself is immunogenic, did not promote GALT development; hence, GALT development in rabbits does not appear to be the result of an Ag-specific immune response. To identify bacterial pathways required for GALT development, we introduced B. fragilis along with stress-response mutants of B. subtilis into germfree-appendix rabbits. We identified two Spo0A-controlled stress responses, sporulation and secretion of the protein YqxM, which are required for GALT development. We conclude that specific members of the commensal, intestinal flora drive GALT development through a specific subset of stress responses.

Immunohistochemical analysis of formaldehyde- and trypsin- or pepsin-treated material

Immunohistochemical studies of paraffin sections of formaldehyde-fixed material and formaldehyde-fixed cryostat sections treated with trypsin or other enzymes were undertaken. Formaldehyde treatment resulted mostly in partial or total loss of antigenicity, as visualized by fluorescent antibody technique in tissue sections. The digestion of sections with trypsin resulted usually in total or partial restitution of antigenicity, previously altered with formaldehyde. This effect is suggested to consist primarily in breaking formaldehyde-induced intermolecular cross-links. Restitution of antigenicity altered by formaldehyde can also be achieved on using pepsin or other proteolytic enzymes, or by some other technique. When the effect of one enzyme proves insufficient, it can readily be complemented by the action of another enzyme applied subsequently. Mild trypsin treatment preserves well the morphological structure of most tissues.

Human appendix B cells naturally express receptors for and respond to interleukin 6 with selective IgA1 and IgA2 synthesis

Past studies have shown that freshly isolated human B cells from peripheral blood and tonsils do not express IL-6 receptors (IL-6R); however, mitogen or antigen activation of these B cells induces IL-6R and responsiveness to IL-6. In this study, we have shown that a high proportion of B cells enzymatically dissociated from human appendix, a gut-associated lymphoreticular tissue (GALT), expresses the IL-6R, and that recombinant human IL-6 induces significant increases in the number of Ig-producing cells. The recombinant human IL-6-induced increase in Ig-producing cells is restricted to the IgA isotype. Further, IgA2 is the major subclass; however, significant numbers of IgA1 producing cells are also seen. In contrast, human tonsillar and peripheral blood B cells express low levels of IL-6R, and exogenous IL-6 does not increase numbers of Ig-producing cells. When PBMC or tonsillar cells are stimulated with PWM, the former display an equal distribution of IgA1 and IgA2 secreting cells, while tonsillar B cells are mainly of the IgA1 subclass. The distribution of surface Ig-positive (sIg+) B cells in the appendix B cell population is sIgA+ greater than sIgG+ greater than sIgM+, and the sIgA+ B cells express higher levels of IL-6R when compared with sIgG+ or sIgM+ B cells. These studies show that human appendix contains B cell subsets that constitutively express IL-6R, and that a high proportion of these cells are committed to the IgA isotype. Furthermore, higher numbers of IL-6 responsive IgA2 B cells are present in the human appendix as compared to tonsils or PBMC.

Neuroimmune appendicitis

BACKGROUND

15-25% of appendices removed from patients with suspected appendicitis appear normal on histological examination. The cause of pain in such patients is unknown. Since the content of neuropeptides seems to be altered in chronic inflammation, we investigated possible changes in peptidergic innervation for substance P (SP), vasoactive intestinal peptide (VIP), and growth-associated protein-43 (GAP-43).

METHODS

Appendices classified as showing acute appendicitis, non-acute appendicitis (clinical signs of acute appendicitis, but histologically not inflamed), or normal were processed for SP, VIP, and GAP-43 immunocytochemistry. The density of SP immunostaining was assessed by digitised morphometry.

FINDINGS

31 appendix specimens were studied (16 acute, 15 non-acute). 16 specimens were used as controls. Expression of GAP-43 was increased in the non-acute appendices. We observed larger amounts of SP-immunoreactive and VIP-immunoreactive nerves in the mucosal layer of the appendix in patients with non-acute appendicitis than in controls and patients with acute appendicitis (mean % area SP-immunoreactive 0.0496 [SD 0.0113] non-acute, 0.0221 [0.0049] acute, 0.0229 [0.0068] controls). In addition, a close spatial relation between SP-immunoreactive and VIP-immunoreactive nerve fibres and lymphoid cells was detected in the outer zone of lymph follicles.

INTERPRETATION

Neuroproliferation in the appendix, in association with an increase in neurotransmitters SP and VIP, may be involved in the pathophysiology of acute right abdominal pain in the absence of an acute inflammation of the appendix. Our data, together with increasing knowledge about the way in which the nervous system and immune cells interact, suggest that neuroimmune appendicitis is a distinct pathological entity.

Gut associated lymphoid tissue: a morphological and immunocytochemical study of the human appendix

Gut associated lymphoid tissue in 15 normal appendices has been characterised in tissue sections using both morphological criteria and immunocytochemical techniques. A panel of monoclonal and polyclonal antibodies was used including antibodies to B-cells, T-cells, macrophages, HLA DR and immunoglobulins. The lymphoid tissue in the appendix was shown to bear a strong resemblance to that in lymph nodes with the exception of the region where the appendix follicles associate with the dome epithelium, which has no lymph node equivalent. This zone of cells between the lymphoid follicles and the dome epithelium termed the 'mixed cell zone' has been shown to contain an abundance of HLA DR-bearing cells, some of which have irregular nuclear morphology and resemble follicle centre cells. These cells were seen to extend into the epithelium of the dome but not the crypts. Using a monoclonal anti-B-cell antibody a population of B-cells was detected in the equivalent areas of mixed cell zone and epithelium and quantitative studies showed that these intraepithelial B-cells comprised approximately 4-5% of the cells in the epithelium. The mixed cell zone was also seen to contain T-cells, S-100 protein-containing macrophages and occasional lysozyme-containing macrophages. Plasma cells were rarely seen in this area.

Intestinal microflora and diversification of the rabbit antibody repertoire

The rabbit establishes its primary Ab repertoire by somatically diversifying an initial repertoire that is limited by restricted VH gene segment usage during VDJ gene rearrangement. Somatic diversification occurs in gut-associated lymphoid tissue (GALT), and by about 1-2 mo of age nearly all Ig VDJ genes are somatically diversified. In other species that are known to establish their primary Ab repertoire by somatic diversification, such as chicken, sheep, and cattle, diversification appears to be developmentally regulated: it begins before birth and occurs independent of exogenous factors. Because somatic diversification in rabbit occurs well after birth in GALT, the diversification process may not be developmentally regulated, but may require interaction with exogenous factors derived from the gut. To test this hypothesis, we examined Ab repertoire diversification in rabbits in which the appendix was ligated shortly after birth to prevent microbial colonization and all other organized GALT was surgically removed. We found that by 12 wk of age nearly 90% of the Ig VDJ genes in PBL were undiversified, indicating that intestinal microflora are required for somatically diversifying the Ab repertoire. We also examined repertoire diversification in sterilely derived remote colony rabbits that were hand raised away from contact with conventional rabbits and thereby acquired a different gut microflora. In these remote colony rabbits, GALT was underdeveloped, and 70% of the Ig VDJ genes in PBL were undiversified. We conclude that specific, currently unidentified intestinal microflora are required for Ab repertoire diversification.

Bacterial translocation in the normal human appendix parallels the development of the local immune system

Experimental modes and pathological conditions may result in bacterial translocation (BT), that is, the passage of indigenous bacteria colonizing the intestine through the intestinal mucosa to mesenteric lymph nodes. Yet no data are available on BT in the normal human gut. We determined the occurrence of BT and its extent in histologically normal, incidentally removed human vermiform appendices (VA) from individuals of different ages and correlated the findings with the development with age of associated lymphatic tissue. BT appears to pertain to normal antigen-sampling processes of the GALT in the VA. It also parallels the development of the GALT and its maintenance during adulthood. In the first two weeks after birth, when bacterial colonization of the gut evolves and when the VA lacks the protection of secretory IgA, BT was not detected. Thereafter, BT occurs along with development of the local GALT, which is fully built up after the first year. A physiological uptake of, or invasion by, bacteria may be instrumental (1) for tolerance induction against the indigenous flora and (2) for the stimulation and normal development of the GALT.

CD5 is a potential selecting ligand for B cell surface immunoglobulin framework region sequences

In rabbits nearly all B lymphocytes express the glycoprotein CD5, in contrast to mice and humans, where only a small proportion of B cells express this molecule (Raman, C., and K.L. Knight. 1992. J. Immunol. 149:3858-3864). CD5+ B cells appear to develop early in ontogeny and be maintained throughout life by self-renewal. The function of CD5 on B cells is still unknown. We showed earlier that "positive" selection occurs during B lymphocyte development in the rabbit appendix. This selection favors B cell expressing surface immunoglobulins with VHa2 structures in the first and third framework regions (Pospisil, R., G.O. Young-Cooper, and R.G. Mage. 1995. Proc. Natl. Acad. Sci. USA. 92:6961-6965). Here we report that F(ab')2 fragments, especially those bearing VHa2 framework region determinants, specifically interact with the B cell-surface glycoprotein CD5. This interaction can be inhibited by anti-CD5 antibodies. Furthermore, immobilized F(ab')2 fragments selectively bind CD5 molecules in appendix cell lysates. Interactions of VH framework region structures with CD5 may affect maintenance and selective expansion of particular B cells and thus contribute to autostimulatory growth of autoimmune or transformed cells.

Development of dome epithelium in gut-associated lymphoid tissues: association of IgA with M cells

The dome epithelium (DE), which covers gut-associated lymphoid tissues (GALT) and provides both a protective barrier over lymphoid follicles and a route for antigen uptake from the gut, develops in rabbit appendix (caecum) during the first week of neonatal life. To determine if secretory immunoglobulins from maternal milk interact with this developing tissue, their interrelationships in neonatal rabbit appendix were examined by use of immunocytochemical techniques. The glycoprotein, secretory component, was not produced by neonatal rabbits less than 15 days old, since neither the membranous nor the free, secreted forms of maternal secretory component were associated with villi or DE of neonates. Immunoglobulin A (IgA), but neither IgG nor IgM, were noted on DE by light microscopy, even though IgG was abundant in the villus lamina propria and vascular spaces. The epithelial IgA was distributed, in a patchy pattern, across the upper dome surface of some two-day-old, and all five-and ten-day old nursing animals, but IgA was not on DE of rabbits prevented from nursing. Immuno-electron microscopy of appendix from nursed rabbits revealed IgA directly over the apical surface of M cells, where it formed a continuous, thick coating without binding to adjacent immature absorptive cells; it was also within apical vacuoles of M cell cytoplasm. The distribution of IgA on the DE of rabbit appendices indicated that in differentiating GALT, maternal IgA reacted preferentially with M cells or pre-M cells, leading to speculation concerning a role for IgA in the development of GALT and in establishment of mucosal immune responses in neonates.

Structure, lymphatic vascularization and lymphocyte migration in mucosa-associated lymphoid tissue

In this review, we consider the morphological aspects and topographical arrangement of gut-associated lymphoid tissue (GALT) (solitary and aggregate lymph nodules or Peyer's patches) and of vermiform appendix in the human child and in some mammals. The spatial arrangement of the vessels belonging to apparatus lymphaticus periphericus absorbens (ALPA) and of blood vessels within each lymphoid follicle as well as the ultrastructural characteristics of the lymphatic endothelium with high absorption capacity are considered. Particular attention is also paid to the morphological and biomolecular mechanisms inducing lymphocyte transendothelial migration to the bloodstream by means of lymphatic vessels as well as their passage from blood into lymphoid tissue through the high endothelial venules (HEVs). The preferential transendothelial passage of lymphocytes and polymorphonuclear neutrophils within ALPA vessels of the interfollicular area does not occur following the opening of intercellular contacts, but rather it occurs by means of 'intraendothelial channels'. In HEVs, on the contrary, the hypothesis is plausible that lymphocyte transendothelial migration into lymphoid tissue occurs through a channel-shaped endothelial invagination entirely independent of interendothelial contacts. The lymph of ALPA vessels of the single Peyer's patch is conveyed into precollector lymphatic vessels and into prelymph nodal collectors, totally independent of the ALPA vessels of the gut segments devoid of lymphoid tissue. The quantitative distribution of T lymphocytes in the lymph of mucosal ALPA vessels suggests a prevalent function of fluid uptake, whereas a reservoir and supply function is implicated for the vessels of interfollicular area. The precollector lymphatic vessels and prelymph nodal collectors are considered to be vessels with low absorption capacity, whose main function is lymph conduction and flow.

Expression of HLA-DR antigens on epithelium associated with lymphoid tissue in the human gastrointestinal tract

Mucosa from human stomach, terminal ileum, appendix, and colon was studied for epithelial HLA-DR expression using an immunoperoxidase technique with a monoclonal antibody that detects HLA-DR antigens in paraffin embedded tissues. Expression of HLA-DR by epithelial cells was studied with particular reference to the effect of adjacent lymphoid tissue or surrounding chronic inflammation. In the stomach, epithelial HLA-DR appeared to be induced by chronic inflammation. Where lymphoid nodules were present only that epithelium directly adjacent to the lymphoid tissue expressed HLA-DR. Expression was independent of cell type. Epithelium adjacent to normal lymphoid tissue in the terminal ileum, appendix, and colon also expressed HLA-DR, the relationship between expression and proximity to lymphoid tissue being remarkably precise. Expression of HLA-DR by gastrointestinal epithelium appears to be an effect of adjacent lymphocytes, whether part of an inflammatory response or normal tissue. This must be taken into account when assessing HLA-DR expression by gastrointestinal epithelium.

Appendiceal inflammation in ulcerative colitis

AIMS

Previous uncontrolled reports have suggested that appendiceal inflammation may occur as a discontinuous lesion in ulcerative colitis. This study aims semiquantitatively to compare the prevalence and histological features of appendiceal inflammation in patients with ulcerative colitis and Crohn's disease, using colonic carcinoma and acute appendicitis specimens as controls.

METHODS AND RESULTS

Surgical pathology records and original histological slides for the period 1980-1994 were examined. The prevalence of appendiceal inflammation in ulcerative colitis (24/50, 48%), was higher than in colonic carcinoma (5/65, 8%, P < 0.001), but was similar to that in Crohn's disease (14/27, 52%). Appendiceal inflammation with caecal sparing was seen in nine out of 24 specimens with ulcerative colitis (37%), two out of nine (22%) with Crohn's disease and five out of 65 (8%) with colonic carcinoma. Inflamed appendixes from patients with inflammatory bowel disease showed histological features typical of ulcerative colitis and Crohn's disease rather than acute appendicitis and were significantly less likely to have transmural inflammation. There had been a previous appendicectomy in 3% of ulcerative colitis patients compared with 8% of colonic carcinoma specimens and 21% (P < 0.01) Crohn's disease controls.

CONCLUSION

In ulcerative colitis, as in Crohn's disease, appendiceal inflammation commonly occurs as a skip lesion and histologically resembles the colonic disease rather than acute appendicitis. The low prevalence of appendicectomy supports the hypothesis that the appendix itself may have a central role in the pathogenesis of ulcerative colitis.

Peyer's patches: immunologic studies

The immune capabilities of the Peyer's patches have been investigated by the use of an in vitro system. Despite our failure to stimulate Peyer's patch lymphocytes in vivo it appears that Peyer's patches behave immunologically as peripheral lymphoid tissues. Cultures prepared from the dissociated Peyer's patches of normal rabbits respond to sheep erythrocytes. The response is comparable to that obtained with spleen cultures from the same animals and is not dependent on the presence of the epithelial cells which line the lumen. Similar thymic cultures do not respond. Our experiments with cultures prepared from rabbits which have received one or two injections of SRC show that the Peyer's patches contain both IgM and IgG "memory" cells which have migrated from the spleen. The concentration of these cells in the spleen remains several hundredfold higher.

Distribution of immunoglobulin producing cells is different in normal human appendix and colon mucosa

The densities of IgG-, IgA-, IgM- and IgD-producing immunocytes were determined by paired immunofluorescence staining and morphometric analysis in the lamina propria of normal appendix specimens. Normal colon specimens were used as reference material, mostly paired from individual subjects. The density (median of cells/mm2 lamina propria area) of IgA immunocytes tended to be slightly higher in the appendix than in the colon (1259 vs 962) and the same held true for IgM cells (71 vs 55). Conversely, the overall density of IgG immunocytes was much higher in the appendix than in the colon (95 vs 38). A striking feature was the fact that almost 50% of all immunocytes were of the IgG isotype adjacent to lymphoid follicles. It seemed justified to conclude, therefore, that the abundance of such follicles explains the overall enrichment of IgG-producing cells in normal appendix mucosa. These immunocytes most likely represent follicle derived B cells that have reached terminal maturation locally, whereas precursors generated from less mature memory clones probably emigrate and home ubiquitously to distant sites of the gut lamina propria where they develop into IgA-producing immunocytes.

B-cell mitogenic effects on human lymphocytes of rabbit anti-human beta 2-microglobulin

Rabbit Ig against human beta2-microglobulin was found to be mitogenic for human peripheral lymphocytes, tonsil lymphocytes, and appendix and spleen cells. Anti-beta2-m gave increased DNA synthesis, with peak responses on day 3 or 4 and showed a dose-response effect when diluted. The effect was seen in both serum-free and serum-containing culture medium. Anti-beta2-m, as well as lipopolysaccharide, induced polyclonal antibody production and intracellular immunoglobulin synthesis in blast cells, which is taken as evidence that these substances are human B-cell mitogens. Anti-beta2-m, but not lipopolysaccharide, could, in these experiments, activate peripheral blood lymphocytes, in addition to lymphoid cells from other sources. Thus, anti-beta2-m can serve as a functional marker for peripheral human B lymphocytes.

Age-dependent variation in the proportion and number of intestinal lymphocyte subsets, especially natural killer T cells, double-positive CD4+ CD8+ cells and B220+ T cells, in mice

The age-dependent variation in the proportion and number of lymphocyte subsets was examined at various extrathymic sites, including the liver, small intestine, colon and appendix in mice. In comparison with young mice (4 weeks of age), the number of total lymphocytes yielded by all tested organs was greater in adult (9 weeks) and old (40 weeks) mice. The major lymphocyte subset that expanded with age was interleukin-2 receptor (IL-2R) beta+ CD3int cells (50% of them expressed NK1.1) in the liver, whereas it was CD3+ IL-2Rbeta- NK1.1- cells at all intraepithelial sites in the intestine. Although NK1.1+ CD3+ cells were present at intraepithelial sites in the intestine, the proportion of this subset was rather low. The ratio of CD4 to CD8 tended to decrease among natural killer T (NKT) cells and T cells at all intraepithelial sites in the intestine with age. A unique population of double-positive CD4+ CD8+ cells in the small intestine increased in old mice. B220+ T cells were found mainly in the appendix and colon, and the proportion of these T cells decreased in old mice. Conventional NKT cells were very few in Jalpha281-/- and CD1d-/- mice in the liver, while NKT cells which existed in the appendix remained unchanged even in these mice. This was because unconventional CD8+ NKT cells were present in the intestine. The present results suggest that despite the fact that both the liver and intraepithelial sites in the intestine carry many extrathymic T cells, the distribution of lymphocyte subsets and their age-associated variation are site-specific.

Preferential expansion and survival of B lymphocytes based on VH framework 1 and framework 3 expression: "positive" selection in appendix of normal and VH-mutant rabbits

B cells with a rearranged heavy-chain variable region VHa allotype-encoding VH1 gene segment predominate throughout the life of normal rabbits and appear to be the source of the majority of serum immunoglobulins, which thus bear VHa allotypes. The functional role(s) of these VH framework region (FR) allotypic structures has not been defined. We show here that B cells expressing surface immunoglobulin with VHa2 allotypic specificities are preferentially expanded and positively selected in the appendix of young rabbits. By flow cytometry, a higher proportion of a2+ B cells were progressing through the cell cycle (S/G2/M) compared to a2- B cells, most of which were in the G1/G0 phase of the cell cycle. The majority of appendix B cells in dark zones of germinal centers of normal 6-week-old rabbits were proliferating and very little apoptosis were observed. In contrast, in 6-week-old VH-mutant ali/ali rabbits, little cell proliferation and extensive apoptosis were observed. Nonetheless even in the absence of VH1, B cells with a2-like surface immunoglobulin had developed and expanded in the appendix of 11-week-old mutants. The numbers and tissue localization of B cells undergoing apoptosis then appeared similar to those found in 6-week-old normal appendix. Thus, B cells with immunoglobulin receptors lacking the VHa2 allotypic structures were less likely to undergo clonal expansion and maturation. These data suggest that "positive" selection of B lymphocytes through FR1 and FR3 VHa allotypic structures occurs during their development in the appendix.

Appendectomy and Clostridium difficile colitis: Relationships revealed by clinical observations and immunology

Advances in understanding the interaction between the human immune system and the microbiome have led to an improved understanding of the function of the vermiform appendix as a safe-house for beneficial bacteria in the colon. These advances have been made despite long standing clinical observations that the appendectomy is a safe and effective procedure. However, more recent clinical data show that an appendectomy puts patients at increased risk for recurrent Clostridium difficile (C. difficile)-associated colitis, and probably other diseases associated with an altered microbiome. At the same time, appendectomy does not apparently put patients at risk for an initial onset of C. difficile-associated colitis. These clinical observations point toward the idea that the vermiform appendix might not effectively protect the microbiome in the face of broad spectrum antibiotics, the use of which precedes the initial onset of C. difficile-associated colitis. Further, these observations point to the idea that historically important threats to the microbiome such as infectious gastrointestinal pathogens have been supplanted by other threats, particularly the use of broad spectrum antibiotics.

Mucin-related epitopes distinguish M cells and enterocytes in rabbit appendix and Peyer's patches

The biochemical composition of the apical membranes of epithelial M cells overlying the gut-associated lymphoid tissues (GALT) is still largely unknown. We have prepared monoclonal antibodies (MAbs) directed against carbonate-washed plasma membranes from epithelial cells detached with EDTA from rabbit appendix, a tissue particularly rich in GALT. As determined by immunofluorescence microscopy, several MAbs specifically recognized either M cells or enterocyte-like cells of the domes from rabbit appendix, sacculus rotundus, and Peyer's patches. M cells were identified by their large ventral pocket containing lymphoid cells and by specific labeling with antivimentin. Among various characterized MAbs, MAb 104 recognized rabbit immunoglobulins and was used as an apical marker for M cells in the rabbit appendix, MAb 58 selectively stained an integral membrane glycoprotein of greater than 205 kDa located at the apex of M cells, and MAb 214 stained a smaller soluble glycoprotein associated with the apical surfaces from neighboring enterocytes. In addition, both MAbs 58 and 214 also labeled luminal mucus and secretory granules in goblet cells. The selective association of mucin-related molecules at the surfaces of either M cells or enterocyte-like cells of the follicle-associated epithelium suggests that specific carbohydrate antigens are differentially expressed by epithelial cells and could account for the differential binding properties of pathogens.

Cells involved in the immune response. IV. The response of normal and immune rabbit bone marrow and lymphoid tissue lymphocytes to antigens in vitro

Cell suspensions of immune rabbit lymph nodes and spleen were capable of undergoing blastogenesis and mitosis and of incorporating tritiated thymidine when maintained in culture with the specific antigen in vitro. They did not respond to other, non-cross-reacting antigens. The blastogenic response obtained with immune lymph node cells could be correlated with the antibody synthesizing capacity of fragment cultures prepared from the same lymph nodes. Cell suspensions of immune bone marrow responded to non-cross-reacting antigens only whereas cell suspensions of immune thymus, sacculus rotundus, and appendix did not respond when exposed to any of the antigens tested. On the other hand, neither fragments nor cell suspensions prepared from lymph nodes, spleen, and thymus of normal, unimmunized rabbits responded with antibody formation and blastogenesis when exposed to any of the antigens. However, normal bone marrow cells responded with marked blastogenesis and tritiated thymidine uptake. The specificity of this in vitro bone marrow response was demonstrated by the fact that the injection of a protein antigen in vivo resulted in the loss of reactivity by the marrow cell to that particular antigen but not to the other, non-cross-reacting antigens. Furthermore, bone marrow cells of tolerant rabbits failed to respond to the specific antigen in vitro. It was also demonstrated that normal bone marrow cells incubated with antigen are capable of forming antibody which could be detected by the fluorescent antibody technique. This response of the bone marrow cells has been localized to the lymphocyte-rich fraction of the bone marrow. It is concluded that the bone marrow lymphocyte, by virtue of its capacity to react with blastogenesis and mitosis and with antibody formation upon initial exposure to the antigen, a capacity not possessed by lymphocytes of the other lymphoid organs, has a preeminent role in the sequence of cellular events culminating in antibody formation.

Neonatal appendectomy impairs mucosal immunity in rabbits

We compared the effects of neonatal appendectomy in rabbits on total Ig and antigen (Ag)-specific Ig levels in the serum and gut, and on plasma cell numbers in the small intestine in response to intraperitoneal (i.p.) and intraduodenal (i.d.) immunizations with ovalbumin (OVA). Animals were sacrificed after 9 weeks. Antibodies (Abs) in the duodenum were collected and quantified by enzyme-linked immunosorbent assay (ELISA) while plasma cells were quantified by double immunofluorescent staining. Appendectomy markedly reduced total intestinal IgA (P < 0.0006), IgM (P < 0.003), and IgG (P < 0.05) relative to controls, whereas total serum Ig levels were not lowered significantly. Moreover, appendectomy nearly ablated OVA-specific IgA (P < 0.007) in the gut and severely depleted OVA-specific IgG in the gut (P < 0.03) and serum (P < 0.007). The sharp decreases in total IgA and anti-OVA IgA were paralleled by decreases in total IgA+ plasma cells (P < 0.0005) and OVA-reactive IgA+ plasma cells (P < 0.05). These results support a major role of the rabbit appendix in seeding the intestinal lamina propria with plasma cell precursors, especially those producing IgA.

A morphological and immunohistological study of the human and rabbit appendix for comparison with the avian bursa

Diversification of the primary antibody repertoire occurs in young rabbit appendix. As a prelude to molecular investigation of whether human appendix has a similar role, we compared the lymphoid morphology and distribution of common B- and T-cell subsets in frozen and/or paraffin-embedded normal appendix specimens at various ages. IgA, IgM and IgG staining patterns were similar in frozen human and rabbit appendices. The elongated follicles of the young human and rabbit appendices regressed with age to resemble Peyer's patches. Although similar in morphology to the bursa, human and rabbit appendix follicles differ in that they do not involute completely with age and contain significant numbers of germinal center (GC) T cells although the number is low early in life. If the human appendix functions as a primary lymphoid organ, it may occur during the first few months of age when the GC T-cell density is low.

Microbial induction of B and T cell areas in rabbit appendix

Gut-associated lymphoid tissue (GALT) development requires interaction with the intestinal microbiota. Because murine secondary lymphoid tissue development is driven by positive feedback interactions between B cells and stromal cells, we used in situ hybridization to determine whether intestinal commensals influence such interactions during rabbit appendix development. The features of positive feedback interactions we examined (CXCL13 mRNA expression, B cell accumulation and FDC differentiation) increased during early follicle development, but stalled in the absence of intestinal commensals. These features were reinitiated by commensals that stimulated follicle development and intrafollicular B cell proliferation. Our results suggest that rabbit appendix follicles develop in two phases: an initial phase of B cell recruitment to nascent follicles, possibly through positive feedback interactions, and a subsequent phase of intrafollicular B cell proliferation stimulated by intestinal commensals. In addition, we found that intestinal commensals stimulate appendix CCL21 mRNA expression and T cell area formation.

Allotype suppression in the rabbit. I. The ontogeny of cells bearing immunoglobulin of paternal allotype and the fate of these cells after treatment with antiallotype antisera

The ability to distinguish immunoglobulin (Ig) of paternal allotype both on lymphocyte membranes and in the serum of neonatal b(4)b(5) heterozygous rabbits has allowed us to study the postnatal ontogeny of cells bearing endogenously synthesized Ig that could not have been derived from the mother. In normal b(4)b(5) rabbits, such endogenously synthesized Ig of paternal allotype is present on the membranes of bone marrow-derived (B) lymphocytes from birth, but does not appear as detectable circulating Ig until approximately 2 wk of age. In the neonate, cells bearing the paternal allotype are potential targets for the induction of chronic allotype suppression. Within 24 h of exposure to anti-allotype antisera in vivo, Ig of the suppressed paternal allotype is no longer detectable on the surface of neonatal lymphoid cells. Further, this lymphocyte membrane Ig is eliminated and not simply masked by the suppressing antibodies. Finally, cells bearing the suppressed allotype are deleted from all lymphoid organs for the duration of total allotype suppression, and reappear first in bone marrow and peripheral blood at the time of spontaneous escape from suppression.