Memory B cells express a phenotype consistent with migratory competence after secondary but not short-term primary immunization

How One Thing Led to Another

I started research in high school, experimenting on immunological tolerance to transplantation antigens. This led to studies of the thymus as the site of maturation of T cells, which led to the discovery, isolation, and clinical transplantation of purified hematopoietic stem cells (HSCs). The induction of immune tolerance with HSCs has led to isolation of other tissue-specific stem cells for regenerative medicine. Our studies of circulating competing germline stem cells in colonial protochordates led us to document competing HSCs. In human acute myelogenous leukemia we showed that all preleukemic mutations occur in HSCs, and determined their order; the final mutations occur in a multipotent progenitor derived from the preleukemic HSC clone. With these, we discovered that CD47 is an upregulated gene in all human cancers and is a "don't eat me" signal; blocking it with antibodies leads to cancer cell phagocytosis. CD47 is the first known gene common to all cancers and is a target for cancer immunotherapy.

Distal IgA immunity can be sustained by alphaEbeta7+ B cells in L-selectin-/- mice following oral immunization

Understanding the role of homing receptors could aid vaccine strategies for developing distal mucosal immunity. Infection studies have revealed that immune intestinal B cells use alpha(4)beta(7) homing receptors, but their role in subsequent oral immunization with soluble antigens is unknown. To assess the influence of L-selectin and alpha(4)beta(7) on distal B cells following oral cholera toxin (CT) immunization, L-selectin-deficient (L-Sel(-/-)) IgA anti-CT-B-specific B cells were enhanced 30-, 9.2-, and 3.5-fold in head and neck lymph nodes (HNLNs), nasal-associated lymphoid tissue, and nasal passages (NPs), respectively, vs. L-Sel(+/+) mice. Cell-sorted intestinal and NP IgA antibody-forming cells (AFCs) were mostly alpha(4)beta(7)(+), unlike HNLN L-Sel(-/-) IgA and IgG anti-CT-B AFCs that were alpha(E)beta(7)(+), contrasting with L-Sel(+/+) HNLN IgA AFCs that were mostly alpha(4)beta(7)(+). In vitro studies revealed that L-Sel(-/-) HNLN B cells preferentially expressed alpha(E) following polyclonal stimulation. These studies show that HNLN B cells express alpha(E)beta(7) in the absence of L-selectin to sustain distal IgA responses.

Transcriptional profiling of antigen-dependent murine B cell differentiation and memory formation

Humoral immunity is characterized by the generation of Ab-secreting plasma cells and memory B cells that can more rapidly generate specific Abs upon Ag exposure than their naive counterparts. To determine the intrinsic differences that distinguish naive and memory B cells and to identify pathways that allow germinal center B cells to differentiate into memory B cells, we compared the transcriptional profiles of highly purified populations of these three cell types along with plasma cells isolated from mice immunized with a T-dependent Ag. The transcriptional profile of memory B cells is similar to that of naive B cells, yet displays several important differences, including increased expression of activation-induced deaminase and several antiapoptotic genes, chemotactic receptors, and costimulatory molecules. Retroviral expression of either Klf2 or Ski, two transcriptional regulators specifically enriched in memory B cells relative to their germinal center precursors, imparted a competitive advantage to Ag receptor and CD40-engaged B cells in vitro. These data suggest that humoral recall responses are more rapid than primary responses due to the expression of a unique transcriptional program by memory B cells that allows them to both be maintained at high frequencies and to detect and rapidly respond to antigenic re-exposure.

CD21 and CD62L shedding are both inducible via P2X7Rs

Neutrophils and lymphocytes are recruited to sites of inflammation and require the adhesion molecule L-selectin (CD62L) for adherence to endothelial cells. Nucleotides released from activated or dying cells at sites of inflammation can mediate signaling through purinergic receptor family II, resulting in CD62L shedding. Activation of B lymphocytes requires the complement receptor type II (CD21) and at the same time leads to shedding of CD21. Both CD62L and CD21 shedding possibly depends on the same families of proteases. In the present study, we characterized peripheral blood naive and memory cells and neutrophils for CD62L surface expression and analyzed benzoyl-benzoyl triphosphate (BzATP)-induced shedding. BzATP is able to induce CD62L shedding in naive and memory lymphocytes, but not in neutrophils. CD21 shedding can be induced through activation of the B cell receptor (BCR) or with mitogens. Here we show that CD21 is also susceptible to BzATP-induced shedding on peripheral B cells. In addition, using receptor inhibitors, we show that shedding of CD21 and CD62L is mediated via the P2X7R. P2X7R-mediated CD62L and CD21 shedding could occur as a result of extracellular accumulated ATP and may have an influence on leukocyte migrational behavior and BCR-mediated signaling.

In multiple myeloma clonotypic CD38- /CD19+ / CD27+ memory B cells recirculate through bone marrow, peripheral blood and lymph nodes

It is believed that myeloma cells are derived from a germinal center (GC) or post GC B cell. The GC B cell can differentiate into both a memory B cell and a plasma cell (PC). In this study, we investigated the recirculating potential of memory B cells clonally related to the myeloma PC (termed clonotypic). The V(H)DJ(H) immunoglobulin gene rearrangement of the myeloma clone was identified for 10 myeloma patients and allele-specific oligonucleotides (ASO) IgH RT-PCR assays were designed for each patient. Memory B cells (CD38- /CD19+ /CD27+) and their subsets defined by the monoclonal antibodies CD62L, CCR6, CXCR4, CXCR5, CCR7 were flow-sorted as single cells and analyzed by ASO RT-PCR analysis. In addition, aspirated peripheral lymph nodes (PLN) of 7 myeloma patients in complete or partial remission were analyzed for the presence of clonotypic cells. Circulating clonotypic memory B cells were identified in PBMNC of 7/10 patients and both CD62L positive and negative clonotypic memory B cells were identified. Furthermore, comparable frequencies of clonotypic cells were found in the CCR6 +/- and CXCR4 +/- memory B cell subsets, whereas all clonotypic memory and later stage B cells were CXCR5 positive. In accordance with their immunophenotype, clonotypic memory B-cells were identified in peripheral blood, bone marrow and PLNs. Clonotypic memory B-cells were present in the majority of myeloma patients and seem to have the same diverse recirculating/homing capacity as normal memory B cells.

Absence of L-selectin delays mucosal B cell responses in nonintestinal effector tissues

Previous studies suggest that lymphocyte trafficking to head and neck lymph nodes, also referred to as cranial-, oral-, nasal-associated lymphoid tissue (CONALT), is L-selectin (L-Sel) dependent, despite coexpression of alpha(4)beta(7), resulting in their marked reduction in L-Sel-deficient (L-Sel(-/-)) mice. Consequently, early phase (16 days) Ab responses to cholera toxin (CT) are diminished. The following studies reveal that lack of mucosal effector responses is not caused by loss of inductive immune responses in the L-Sel(-/-) CONALT. Indeed, there was an increased accumulation of total IgA, but not Ag-specific IgA Ab-forming cells (AFC) in L-Sel(-/-) CONALT. This increased accumulation was not evident in L-Sel(+/+) CONALT. Identification of lymphocyte-homing receptors on L-Sel(-/-) and L-Sel(+/+) CONALT lymphocytes revealed no significant differences in expression of alpha(4)beta(7), which might contribute to lymphocyte homing in the absence of L-Sel. Studies of CONALT responses during the late phase (6 wk post-intranasal immunization) revealed the number of lymphocytes recovered from L-Sel(-/-) CONALT was less than L-Sel(+/+) CONALT; however, L-Sel(-/-) CT-specific and total AFC did not vary from 16-day responses, suggesting a defect in CT-specific B cell export. No significant differences in alpha(4)beta(7) expression between L-Sel(-/-) and L-Sel(+/+) CONALT were noted. Yet, these increases in CONALT AFC correlated with restoration of immunity in L-Sel(-/-) nasal passages and reproductive tracts.

Cytokine control of memory B cell homing machinery

The germinal center (GC) is a pivotal site for the development of B cell memory. Whereas GC B cells do not chemotax to most chemokines and do not express the adhesion receptors L-selectin, alpha(4)beta(7), and cutaneous lymphocyte Ag (CLA), memory B cells respond to various chemotactic signals and express adhesion receptors. In this study, we show that CD40 ligand, IL-2, and IL-10 together drive this transition of GC B cells to memory phenotype in vitro, up-regulating memory B cell markers, chemotactic responses to CXC ligand (CXCL)12, CXCL13, and CCL19, and expression of adhesion receptors L-selectin, alpha(4)beta(7), and CLA. Moreover, addition of IL-4 modulates this transition, preventing chemotactic responses to CXCL12 and CXCL13 (but not to CCL19), and inhibiting the re-expression of L-selectin, but not of CLA or alpha(4)beta(7). CCR7 expression, responsiveness to CCL19, and L-selectin/alpha(4)beta(7) phenotype are coordinately regulated. Thus, IL-2/IL-10 and IL-4 play important and distinctive roles in developing the migratory capacities of memory B cells.

Alterations in Cell Surface Carbohydrates on T Cells from Virally Infected Mice Can Distinguish Effector/Memory CD8+ T Cells from Naive Cells

Glycosylation changes on surface molecules of T cells affect cell trafficking and function and may be useful in discriminating between naive, effector, and memory T cells. To analyze oligosaccharide structures on T cells activated in vivo, we examined alterations in sialic acid residues on T cells following infection of mice with lymphocytic choriomeningitis (LCMV), vaccinia virus, and vesicular stomatitis virus. We found that the majority of CD8 T cells from mice acutely infected with these viruses showed increased binding to peanut agglutinin (PNA). All of the PNAhighCD8 T cells from infected mice were CD44high, indicating that glycosylation changes were occurring on activated T cells. There was also an increase in the PNAhighCD4 T cell population in virally infected mice. Increased PNA binding to activated CD8 T cells correlated with higher endogenous neuraminidase levels in these cells. This higher neuraminidase activity most likely contributed to the PNAhigh phenotype by cleaving sialic acid residues off the core-1 O-glycans or glycoproteins destined for the cell surface. A PNAhighCD8 T cell population persisted in immune mice that had cleared the LCMV infection. When spleen cells from immune mice were sorted into PNAhigh and PNAlow populations, >95% of the LCMV-specific memory CD8 T cells segregated with the PNAhigh population. This shows that virus-specific memory CD8 T cells remain hyposialylated and can be distinguished from naive CD8 T cells based on PNA binding. Thus, PNA can be used as a marker for Ag-experienced T cells.

Normal and clonal B lineage cells can be distinguished by their differential expression of B cell antigens and adhesion molecules in peripheral blood from multiple myeloma (MM) patients--diagnostic and clinical implications

Human MM is a haematologic disorder characterized by the accumulation of malignant plasma cells (PC), primarily in the bone marrow (BM). Although these cells characteristically home to the BM, in recent years several groups have detected the presence of related malignant B cells in the peripheral blood (PB) which could be implicated in the progression and spread of the disease. However, the proportion and origin of these clonotypic circulating B cells is still controversial. In this study, using a triple-staining flow cytometric procedure and a whole blood lysis method, PB B lineage cells could be divided into two populations according to their distinct repertoires of cell adhesion molecules and B cell antigens in untreated MM patients. The results show that: (i) the percentage and the absolute number of PB CD19+ B cells were decreased in MM patients compared with controls; (ii) the quantity and percentage of B cell antigens (CD20, CD22, CD24, DR, CD138) and adhesion molecules (beta1- and beta2-integrins, CD44, CD54, CD56, CD61 and CD62L) expressed by these PB CD19+ cells of MM patients and healthy subjects were similar and all of them were virtually polyclonal cells; (iii) a very minor circulating CD19-CD38++CD45-/dim subset was also detected which expressed CD138 (B-B4) (high intensity), monoclonal cytoplasmic immunoglobulin (cIg), and was negative for pan-B antigens (CD19, CD20, CD24, DR), surface immunoglobulin (sIg) and several adhesion molecules such as CD62L, CD18 and CD11a; this CD19-CD38++CD45-/dim CD138++ subset was not found in normal blood and exhibited a phenotypic profile which was closely related to that of malignant BM plasma cells, with the exception of the CD56 antigen. Polymerase chain reaction (PCR) analysis of IgH clonotypic rearrangements confirmed these results. We postulate that, in MM patients, circulating B lineage cells may be divided into two different categories: polyclonal CD19+ B cells and a very minor proportion of clonal CD138++ PC that escape from the BM.

Distinct Recirculating and Non-Recirculating B-Lymphocyte Pools in the Peripheral Blood Are Defined by Coordinated Expression of CD21 and L-Selectin

The continual recirculation of lymphocytes between the blood, tissues, and lymph is essential for the coordination and dissemination of immune responses. We have compared the functional and phenotypic properties of lymphocytes isolated from blood and lymph, the two major migratory populations. Lymph-borne lymphocytes migrated readily into the lymphatic recirculation pathway, but greater than one third of all peripheral blood lymphocytes (PBLs) were excluded from the lymphatic circuit and showed an enhanced migration to the spleen. Phenotypic analysis showed that most non-recirculating PBLs were B cells. The migration competence of B cells correlated with the surface expression of CD21 and L-selectin; recirculating B cells expressed both of these molecules, whereas non-recirculating B cells lacked both antigens. These results establish that blood contains distinct pools of lymphocytes that differ in their recirculation competence. Clearly, blood sampling is not an efficient method to directly measure the status of the recirculating immune system, and implies important constraints and restrictions in the interpretation of experimental or clinical data that include phenotypic and quantitative analyses of blood lymphocytes.

Distinct Recirculating and Non-Recirculating B-Lymphocyte Pools in the Peripheral Blood Are Defined by Coordinated Expression of CD21 and L-Selectin

The continual recirculation of lymphocytes between the blood, tissues, and lymph is essential for the coordination and dissemination of immune responses. We have compared the functional and phenotypic properties of lymphocytes isolated from blood and lymph, the two major migratory populations. Lymph-borne lymphocytes migrated readily into the lymphatic recirculation pathway, but greater than one third of all peripheral blood lymphocytes (PBLs) were excluded from the lymphatic circuit and showed an enhanced migration to the spleen. Phenotypic analysis showed that most non-recirculating PBLs were B cells. The migration competence of B cells correlated with the surface expression of CD21 and L-selectin; recirculating B cells expressed both of these molecules, whereas non-recirculating B cells lacked both antigens. These results establish that blood contains distinct pools of lymphocytes that differ in their recirculation competence. Clearly, blood sampling is not an efficient method to directly measure the status of the recirculating immune system, and implies important constraints and restrictions in the interpretation of experimental or clinical data that include phenotypic and quantitative analyses of blood lymphocytes.

Differential regulation of leucocyte L-selectin (CD62L) expression in normal lymphoid and inflamed extralymphoid tissues

AIMS

To study tissue expression of L-selectin, a leucocyte cell surface molecule that is considered to be involved in adhesion to certain endothelia, particularly in peripheral lymph nodes and during inflammation, and is shed upon leucocyte activation.

METHODS

Leucocytes were examined by immunohistochemistry and double immunofluorescence staining in various lymphoid sites and normal and inflamed extralymphoid tissues.

RESULTS

L-selectin was present on mantle zone B lymphocytes in different lymphoid sites, including in intestinal lymphoid tissue, but was absent on germinal centre B cells. Splenic white pulp B cells also expressed L-selectin. The proportion of T lymphocytes expressing L-selectin depended on the site under study, being greatest in peripheral lymph nodes (mean 48% of T cells positive), and lower in mucosal lymphoid sites and spleen (9 and 11% positive, respectively). Non-lymphocytic L-selectin staining was observed on follicular dendritic cells in tonsils and on macrophages in thymus. L-selectin positive leucocytes were rare in normal extralymphoid tissues, and relatively few were seen in most inflammatory settings. However, in rejecting renal transplants, a higher proportion (30%) of leucocytes expressed L-selectin.

CONCLUSIONS

Overall, the results indicate how the degree of L-selectin expression by leucocytes in particular tissues may reflect a requirement for L-selectin expression for entry into those tissues and the activation state of leucocytes once localised there.

Lack of L-selectin expression by cells transferring diabetes in NOD mice: insights into the mechanisms involved in diabetes prevention by Mel-14 antibody treatment

The process of mononuclear cell extravasation from the blood into the islets of Langerhans in nonobese diabetic (NOD) mice is dependent on the expression of a set of molecules, most of which remain to be defined. The observation that vascular addressins are expressed in inflamed islets raises the issue of the involvement of one of their ligands, L-selectin, in the pathogenesis of autoimmune diabetes. Treatment of NOD females with Mel-14, an antibody specific for L-selectin, reduced the spontaneous development of both insulitis and diabetes. Pretreatment of diabetic donors with Mel-14 decreased the capacity of their splenocytes to transfer the disease. However, the treatment of recipients had no effect on the transfer of diabetes by untreated diabetogenic splenocytes. To reconcile these apparently conflicting results, we fractionated spleen T cells from diabetic mice according to L-selectin expression. Diabetogenic cells were found only in the L-selectin subpopulation. Thus, diabetogenic cells in adult mice share phenotypic characteristics with activated/memory cells, and enter the pancreas using L-selectin-independent migratory pathways.

Interleukin-4 induces in vitro migration in naive B cells through direct mechanisms

Interleukin-4 (IL-4) mediated locomotor responses by murine B cells in vitro were examined in this paper. The IL-4 induced migration was found to act directly on purified, splenic B cells, without involvement of secondary mediators. It appeared that only a subpopulation of B cells was able to respond in migration assays. Flowcytometric analysis showed that the migrating cells had the characteristics of naive B cells: I-Alo, J11dhi, IgDhi. They also displayed high expression of the adhesion molecule L-selectin and made predominantly IgM antibodies. This is contrary to what has previously been observed regarding motile responses to chemotactic factors by T cells, which mostly affect memory or activated T cells. However, this is in accordance with other studies indicating that IL-4 is a cytokine that exerts its effect mainly on resting B cells.

Cell-cell interactions that regulate the development of B-lineage cells

Significant progress has been made recently in our understanding of the functions of lymphocyte-associated surface proteins. The latest developments involve the identification of ligands or co-receptors for many of these surface proteins. The signal transduction mechanisms utilized by these molecules are also beginning to be elucidated.

The influence of dietary fat on the interaction of lymphocytes with high endothelial venules

The specific adherence of lymphocytes to high endothelial venules (HEV) represents the first step in the lymphocyte emigration from blood into most lymphoid tissues. The interaction of lymphocytes with HEV exhibits a remarkable organ specificity, which appears to be mediated by complementary receptors on both recirculating lymphocytes (homing receptors) and tissue-specific HEV (vascular addressins). The expression of homing receptors varies and depends on factors such as lymphocyte subtype, stage of activation and maturation. As these receptors are glycoproteins, which are anchored in the cell membrane, it can be envisaged that their position and function are determined by the overall composition of the cell membrane itself. In this study we investigated the significance of dietary fat concentration and saturation for the interaction between lymphocytes and HEV. In addition to these functional studies, the expression of homing receptors in combination with T and B cell markers were analyzed. Using immunohistochemistry the effect on the presence and characteristics of lymphocytes and HEV in situ was studied. Changes in the dietary fatty acid composition resulted in an altered ability of T and B cells to adhere to HEV, without affecting their binding preference. The changed adhesion patterns seemed to be associated with alterations in the expression of adhesion molecules, that are essential for the lymphocyte migration. The latter observation might in turn be explained by the observed modifications in the fatty acid composition of the lymphocytes. These results suggest a role for the fatty acid composition of the nutrition in the process of lymphocyte recirculation.

Lymphocyte activation and regulation of three adhesion molecules with supposed function in homing: LECAM-1 (MEL-14 antigen), LPAM-1/2 (alpha 4-integrin) and CD44 (Pgp-1)

Directed migration of lymphocytes from blood into lymph nodes and gut-associated lymphatic tissue, also referred to as homing, is subject to change following activation. Lymphocyte migration into lymphoid organs in vivo and binding to high endothelial venules in vitro is largely suppressed after short-term stimulation with phorbol esters. The observed functional alterations were correlated with changes in the expression of three putative homing receptors, LECAM-1 (MEL-14 antigen), LPAM-1/2 (alpha 4-integrin) and the murine CD44 (Pgp-1, H-CAM, Hermes-antigen equivalent) upon different modes of cellular activation. Expression of LECAM-1 (gp90 MEL-14), a lymphocyte adhesion molecule implicated in targeting extravasation into lymph nodes, was found to be lost almost completely within minutes after protein kinase C activation. LECAM-1 re-expression occurred within less than 24 h. Rapid loss of LECAM-1 was also observed after calcium ionophores whereas anti-CD3 or concanavalin A elicited a gradual and heterogeneous loss of LECAM-1 becoming detectable after several hours only. A number of cytokines tested were not able to induce alterations in LECAM-1 expression. In contrast, expression of LPAM-1/2 (alpha 4-integrin) and CD44 (Pgp-1, H-CAM), two adhesion molecules supposed to direct extravasation into Peyer's patches, remained stable for hours after every stimulus tested; CD44 expression gradually increased 24 h after mitogenic activation, whereas a small reduction only was observed for the expression of the alpha 4-chain under certain conditions. Thus, reduced extravasation of lymphocytes into Peyer's patches after activation is not due to a decline in the surface density of LPAM-1/2 alpha-chain or CD44 whereas alterations in migration into lymph nodes parallel the expression of LECAM-1.

Chemotaxis of germinal center B cells in response to C5a

An infiltrate of B cells and plasma cells is characteristic of certain chronic inflammatory lesions. However, mechanisms involved in the local accumulation of these cells have not been established. Efforts to demonstrate that B cells from normal animals can migrate in response to inflammation-induced chemoattractants have been inconclusive. The objective of this study was to determine if murine germinal center (GC) B cells could respond chemotactically to a C5a gradient. On successive days after secondary immunization, draining lymph nodes were harvested and the activated GC B cells isolated. These GC B cells were placed in modified Boyden chambers, incubated for 3 h and the distance the leading front of cells migrated through the filters was determined. The results show that GC B cells migrated to factors in zymosan- and lipopolysaccharide-activated serum. The migratory response demonstrated distinct kinetics. Cells isolated between 2 to 4 days after secondary immunization migrated, whereas cells isolated at day 0 and beyond day 6 did not. Checkerboard analysis revealed that the migratory response was attributable to both chemokinesis and chemotaxis. Anti-C5 inhibited the migration of day-3 GC B cells implicating C5 in the migration mechanism. Studies using recombinant C5a established that this C5 fragment was chemotactically active. In conclusion, GC B cells generally were not chemotactically active. However, at a particular stage of maturation B cells in the GC become responsive to C5a as a chemotactic agent. Thus, B cells from normal animals may respond chemotactically, and C5a may play a role in recruitment of recently activated B cells into inflammatory sites.

Bcl-2 maintains B cell memory

The number of lymphocytes in an animal is remarkably constant despite antigen-driven proliferation and a high rate of B-cell lymphopoiesis. This reflects the relatively brief lifespan of many newly generated B cells and argues for a well-regulated death mechanism. Even so, a secondary immune response can be generated years after a primary exposure to antigen. Antigen that might restimulate B cells persists for extended periods on follicular dendritic cells in the light zone of germinal centres. Antigen-binding B cells have also been found months after the end of obvious cell division. The precise signal that enables certain B cells to emerge as long-term surviving memory cells is unknown. Bcl-2, an inner mitochondrial membrane protein, blocks programmed cell death in B cells. We report here that this proto-oncogene maintains immune responsiveness. Transgenic mice overproducing Bcl-2 have a long-term persistence of immunoglobulin-secreting cells and an extended lifetime for memory B cells.

Lamina propria lymphocytes are derived from circulating cells that lack the Leu-8 lymph node homing receptor

The Leu-8 membrane glycoprotein is the primate homologue of the murine MEL-14 peripheral lymph node homing receptor and is expressed on a majority of circulating lymphocytes but on few lymphocytes in the intestinal lamina propria. To examine the mechanisms regulating expression of the Leu-8-molecule on lymphocytes in different tissue sites, studies of Leu-8 membrane antigen expression, Leu-8 messenger RNA, and the Leu-8 gene were performed using normal human and nonhuman primate lymphocytes. Activation of resting peripheral blood lymphocytes caused a rapid decrease in membrane Leu-8 expression, a more gradual decrease in Leu-8 messenger RNA, and an increase in expression of interleukin 2 and interleukin 2 receptor messenger RNA. However, the down regulation of Leu-8 expression during activation was reversible because both membrane Leu-8 antigen and Leu-8 messenger RNA were reexpressed after 5 days of culture. Leu-8 messenger RNA was present in lymphocytes isolated from peripheral blood, spleen, and, particularly, mesenteric lymph node, but intestinal lamina propria lymphocytes contained very low levels of Leu-8 messenger RNA. The absence of Leu-8 messenger RNA in intestinal lymphocytes and circulating Leu-8 negative lymphocytes was not caused by recent activation in vivo because these cells did not have detectable interleukin 2 messenger RNA, and intestinal lymphocytes did not express Leu-8 after culture in vitro. Phorbol myristate acetate was found to be a strong inducer of Leu-8 messenger RNA in Leu-8-positive lymphocytes; however, phorbol myristate acetate did not induce membrane Leu-8 expression or Leu-8 messenger RNA in lamina propria lymphocytes. Leu-8-negative lymphocytes in peripheral blood or lamina propria did not have evidence of deletion or rearrangement of the Leu-8 gene. Leu-8-positive Jurkat cells and peripheral blood lymphocytes and Leu-8-negative peripheral blood and intestinal lymphocytes had partial methylation of an Msp I site in proximity to the Leu-8 gene, suggesting that in Leu-8-negative lymphocytes, the Leu-8 gene previously was transcriptionally active. In summary, these studies demonstrate that intestinal lamina propria lymphocytes have the same characteristics as circulating Leu-8-negative lymphocytes with respect to their state of activation and inability to express the Leu-8 antigen. The results suggest that the majority of lymphocytes that migrate to the intestinal lamina propria are derived from the subpopulation of circulating Leu-8-negative lymphocytes.

Responses of single germinal-center B cells in T-cell-dependent microculture

B cells purified from the germinal centers (GCs) of murine Peyer's patches can be stimulated in a clonal microculture containing helper T cells and dendritic cells to divide and secrete immunoglobulin. Intraclonal isotype switching occurs, and a variety of immunoglobulin isotypes, including IgA, is secreted. Memory cells, which generate clones secreting IgA exclusively, are only rarely identified in the GC B-cell subset. Such memory cells can, however, be readily identified among unfractionated Peyer's patch B cells, and in non-GC subsets of B cells. The results suggest that the GC does not contain IgA memory cells that can be restimulated in vitro to secrete only IgA. When division of GC B cells is prevented by irradiation or aphidicholin treatment, a large subset that secretes IgA as the sole immunoglobulin isotype is seen, and the output of presumably single B cells is large enough to be scored by RIA. Both helper T cells and dendritic cells are required for the phenomenon. The data indicate that commitment to IgA secretion occurs in Peyer's patch GCs and suggest that the prolific cell division known to be supported in GCs may forestall terminal differentiation of preplasmablasts to immunoglobulin secretion.

Germinal center kinetics in lymph nodes of primed mice stimulated with complexed as opposed to free antigen

Primed mice with low titers of circulating tetanus antitoxin (AB) were stimulated via the hind footpads with either fluid tetanus toxoid alone (AG) to create in vivo complexes in AG excess, or the same dose of toxoid complexed at equivalence with isologous antibody (AB-AG CPX), to have in vivo complexes in AB excess. All experimental animals reacted with three topically distinct consecutive waves of enhanced proliferative activity in popliteal lymph nodes, i.e., in the T-zone (peak on day 2), in the medullary area, the main site of plasmocytopoiesis (day 3), and in lymphoid follicles (day 5-6). Maximum serum AB titers following injection of AG-AB CPX were only about 25% of those found in animals boosted with AG alone. This suppressive effect was best reflected in a comparable reduction in plasmocytopoiesis, and to an lesser extent in the proliferative activity within the T-zone, and not at all in the overall magnitude of germinal center formation and/or expansion. However, the patterns of germinal center kinetics differed markedly between the two groups: a high sharp peak of development on day 5, followed by a marked drop on day 6 characterized the response in mice given AG alone, and a broad peak around day 6 that of those receiving AG-AB CPX. These differences could not adequately be accounted for by variations in centroblast/centrocyte proliferation rate vs. pycnotic indices, so that different patterns of lymphoid cell emigration from the centers may be considered. The results suggest that immune complexes, fixed on follicular dendritic cells, with different antigen-to-antibody ratios have divergent effects on the development and kinetics of germinal centers, the principal sites of memory B cell generation.