Germinal center T cells are distinct helper-inducer T cells

Transcriptome and TCR Repertoire Measurements of CXCR3+ T Follicular Helper Cells Within HIV-Infected Human Lymph Nodes

Follicular-helper T cells (T_FH) are an essential arm of the adaptive immune system. Although T_FH were first discovered through their ability to contribute to antibody affinity maturation through co-stimulatory interactions with B cells, new light has been shed on their ability to remain a complex and functionally plastic cell type. Due to a lack sample availability, however, many studies have been limited to characterizing T_FH in mice or non-canonical tissue types, such as peripheral blood. Such constraints have resulted in a limited, and sometimes contradictory, understanding of this fundamental cell type. One subset of T_FH receiving attention in chronic infection are CXCR3-expressing T_FH cells (CXCR3⁺T_FH) due to their abnormal accumulation in secondary lymphoid tissues. Their function and clonal relationship with other T_FH subsets in lymphoid tissues during infection, however, remains largely unclear. We thus systematically investigated this and other subsets of T_FH within untreated HIV-infected human lymph nodes using Mass CyTOF and a combination of RNA and TCR repertoire sequencing. We show an inflation of the CXCR3⁺T_FH compartment during HIV infection that correlates with a lower HIV burden. Deeper analysis into this population revealed a functional shift of CXCR3⁺T_FH away from germinal center T_FH (GC-T_FH), including the altered expression of several important transcription factors and cytokines. CXCR3⁺T_FH also upregulated cell migration transcriptional programs and were clonally related to peripheral T_FH populations. In combination, these data suggest that CXCR3⁺T_FH have a greater tendency to enter circulation than their CXCR3^- counterparts, potentially functioning through distinct modalities that may lead to enhanced defense.

The receptor repertoire and functional profile of follicular T cells in HIV-infected lymph nodes

Follicular helper CD4⁺ T cells (T_FH) play an integral role in promoting B cell differentiation and affinity maturation. Whereas T_FH cell frequencies are increased in lymph nodes (LNs) from individuals infected with HIV, humoral immunity remains impaired during chronic HIV infection. Whether HIV inhibits T_FH responses in LNs remains unclear. Advances in this area have been limited by the difficulty of accessing human lymphoid tissues. Here, we combined high-dimensional mass cytometry with T cell receptor repertoire sequencing to interrogate the composition of T_FH cells in primary human LNs. We found evidence for intact antigen-driven clonal expansion of T_FH cells and selective utilization of specific complementarity-determining region 3 (CDR3) motifs during chronic HIV infection, but the resulting T_FH cells acquired an activation-related T_FH cell signature characterized by interleukin-21 (IL-21) dominance. These IL-21⁺ T_FH cells contained an oligoclonal HIV-reactive population that preferentially accumulated in patients with severe HIV infection and was associated with aberrant B cell distribution in the same LN. These data indicate that T_FH cells remain capable of responding to HIV antigens during chronic HIV infection but become functionally skewed and oligoclonally restricted under persistent antigen stimulation.

Molecular Control of Follicular Helper T cell Development and Differentiation

Follicular helper T cells (Tfh) are specialized helper T cells that are predominantly located in germinal centers and provide help to B cells. The development and differentiation of Tfh cells has been shown to be regulated by transcription factors, such as B-cell lymphoma 6 protein (Bcl-6), signal transducer and activator of transcription 3 (STAT3) and B lymphocyte-induced maturation protein-1 (Blimp-1). In addition, cytokines, including IL-21, have been found to be important for Tfh cell development. Moreover, several epigenetic modifications have also been reported to be involved in the determination of Tfh cell fate. The regulatory network is complicated, and the number of novel molecules demonstrated to control the fate of Tfh cells is increasing. Therefore, this review aims to summarize the current knowledge regarding the molecular regulation of Tfh cell development and differentiation at the protein level and at the epigenetic level to elucidate Tfh cell biology and provide potential targets for clinical interventions in the future.

Toll-like receptor 3 adjuvant in combination with virus-like particles elicit a humoral response against HIV

Human Immunodeficiency Virus (HIV) Virus-Like Particles (VLPs) composed of HIV_IIIB Gag and HIV_BaL gp120/gp41 envelope are a pseudovirion vaccine capable of presenting antigens in their native conformations. To enhance the immunogenicity of the HIV Env antigen, VLPs were coupled to VesiVax Conjugatable Adjuvant Lipid Vesicles (CALV) containing one of four toll-like-receptor (TLR) ligands, each activating a receptor with distinct cellular localization and downstream pathways. C57BL/6 mice were vaccinated by intranasal prime followed by two sub-cheek boosts and their sera immunoglobulin and neutralizing potency were measured over a duration of 3months after vaccination. PBS control, VLPs alone, CALV+VLPs, and VLPs complexed with CALV and ligands for TLR2 (PAM3CAG), TLR3 (dsRNA), TLR4 (MPLA), or TLR7/8 (resiquimod) were evaluated based on antibody titer, IgG1 and IgG2c class switching, germinal center formation, T follicular cells and potency of neutralizing antibodies. Consistently, the TLR3 ligand dsRNA complexed to CALV and in combination with VLPs (CALV(dsRNA)+VLPs) induced the strongest response. CALV(dsRNA)+VLPs induced the highest titers against the recombinant vaccine antigens clade B Bal gp120 and pr55 Gag. Additionally, CALV(dsRNA)+VLPs induced cross-clade antibodies, represented by high titers of antibody to clade c 96ZM651 gp120. CALV(dsRNA)+VLPs induced predominantly IgG2c over IgG1, a response associated with T helper type 1 (Th1)-like cytokines. In turn, CALV(dsRNA)+VLP immunized mice generated the most potent neutralizing antibodies against HIV strain MN.3. Finally, at time of sacrifice, a significant increase in germinal center B cells and T follicular cells was detected in mice which received CALV(dsRNA)+VLPs compared to PBS. Our results indicate that CALV(dsRNA) is a superior adjuvant for HIV VLPs in generating a Th1-like immunoglobulin profile, while prolonging lymph node germinal centers, T follicular cells and generating neutralizing antibodies to a highly sensitive tier 1A variant of HIV.

Key role of CXCL13/CXCR5 axis for cerebrospinal fluid B cell recruitment in pediatric OMS

To study aberrant B cell trafficking into the CSF in opsoclonus-myoclonus syndrome (OMS), chemoattractants CXCL13 and CXCL12, and B cell frequency and CXCR5 expression, were evaluated. CSF CXCL13 concentration and the CSF/serum ratio were higher in untreated OMS than controls, related directly to OMS severity and inversely to OMS duration, and correlated with CSF B cell frequency and oligoclonal bands. CXCL12 showed the opposite pattern. Selective accumulation of CXCR5+ memory B cells in CSF was found. In ACTH-treated OMS, CXCL13, but not CXCL12, was lower. These data implicate the chemokine/chemoreceptor pair CXCL13/CXR5 in B cell recruitment to the CNS in OMS. CXCL13 and CXCL12 may serve as reciprocal biomarkers of disease activity, but CXCL13 also had utility as a treatment biomarker.

Emerging cellular networks for regulation of T follicular helper cells

The cellular networks that regulate humoral immune responses have been a focus of research over the past three decades. Studies have shown that inhibition of immune responses can be attributed to both suppressor T cells and B cells. More recently, T follicular helper (Tfh) cells have been identified as a target of immune regulation. Tfh cells are a subset of highly activated T helper cells specialized for providing cognate help to B cells during germinal center reactions. In this review, we describe emerging evidence for cellular networks that alter Tfh cell phenotype and function and regulate antibody production during the germinal center reaction. We discuss how these new findings influence our understanding of Tfh cells.

Nodular lymphocyte-predominant hodgkin lymphoma with atypical T cells: a morphologic variant mimicking peripheral T-cell lymphoma

Nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL) is a distinct Hodgkin lymphoma subtype composed of few neoplastic lymphocyte-predominant (LP) cells in a background of reactive small B and T cells. We have seen occasional NLPHL cases that contain background T cells with prominent cytologic atypia, raising the differential diagnosis of peripheral T-cell lymphoma not otherwise specified (PTCL-NOS) or a composite lymphoma. We sought to characterize the clinicopathologic features of such cases. Eleven NLPHL cases with atypical T cells diagnosed from 1977 to 2010 were identified at 2 institutions and compared with 24 control NLPHL cases lacking atypical T cells. All 9 male patients and 2 female patients presented with localized peripheral lymphadenopathy. In comparison with control patients, they were younger (median age, 13.8 vs. 36.1 y; P=0.015), with more frequent cervical lymph node involvement (54.5% vs. 8.3%, P=0.015). In all 11 cases, areas of NLPHL with typical B-cell-rich nodules containing LP cells were present. Nine cases contained sheets of atypical T cells surrounding primary and secondary follicles in a pattern mimicking the T-zone pattern of PTCL-NOS; the remaining 2 cases contained atypical T cells presented as large clusters at the periphery of B-cell-rich nodules. In all cases, the atypical T-cell-rich areas contained rare scattered LP cells, which were IgD in 5 of 7 cases (71.4%). The atypical T cells showed no pan-T-cell antigen loss or aberrant T-cell antigen expression in any case, and polymerase chain reaction or Southern blot analysis showed no evidence of T-cell clonality in 6 cases tested. The atypical T cells exhibited a variable immunophenotype with respect to germinal center, follicular T-helper, T-regulatory, and cytotoxic T-cell markers. Among 8 patients with clinical follow-up (median follow-up: 6.4 y), 5 patients had recurrent NLPHL at 6 months to 12 years after diagnosis and 6 patients are alive without disease at 9 months to 18 years after diagnosis. In comparison with control patients, NLPHL patients with atypical T cells were more likely to develop recurrent NLPHL (71.4% vs. 13.6%, P=0.008) and to have a shorter time to relapse (P=0.04). Our findings suggest that some cases of NLPHL, occurring predominantly in younger patients, contain prominent populations of morphologically atypical T cells that may raise the possibility of concurrent nodal involvement by PTCL-NOS, a rare diagnosis in children. The clinical behavior of these cases appears similar to that of NLPHL with T-cell-rich diffuse areas, with a higher risk of disease recurrence and no difference in overall survival; however, this finding warrants confirmation in studies of larger numbers of patients.

Early viral replication in lymph nodes provides HIV with a means by which to escape NK-cell-mediated control

Acute HIV infection is marked by dramatic viral replication associated with preferential replication within secondary lymphoid tissues, such as lymph nodes (LNs), that is rapidly but incompletely contained to a viral setpoint. Accumulating evidence supports a role for natural killer (NK) cells in the early control of HIV infection; however, little is known about the location of their antiviral control. Given that HIV replicates profusely in LNs during early infection, we sought to define whether changes occurred in the NK cell infiltrate within these sites during the first year of HIV infection. Surprisingly, NK cell numbers and distribution were unaltered during early HIV infection. LN NK cells expressed decreased inhibitory receptors, were more highly activated, and expressed elevated TRAIL, potentially conferring a superior capacity for NK cells to become activated and control infection. Most noticeably, KIR(+) NK cells were rarely detected in the LN during HIV infection, associated with diminished migratory capacity in the setting of reduced expression of CX3CR1 and CXCR1. Thus, incomplete control of HIV viral replication during early disease may be due to the inefficient recruitment of KIR(+) NK cells to this vulnerable site, providing HIV a niche where it can replicate unabated by early NK-cell-mediated innate pressure.

Follicular helper T cells as cognate regulators of B cell immunity

Follicular helper T (T(FH)) cells are a class of helper T cells specialized in the cognate control of antigen-specific B cell immunity. Upon first contact with antigen-primed B cells, pregerminal center effector T(FH) cells promote B cell clonal expansion, antibody isotype switch, plasma cell differentiation, and the induction of germinal centers. By contrast, within germinal centers, T(FH) cells regulate the fate of antigen-specific GC B cells expressing high-affinity variant B cell receptors to promote memory B cell and long-lived plasma cell development. Recent studies unravel multiple signals controlling T(FH) development and functional subtypes of antigen-specific T(FH) cells, including memory T(FH) cells that accelerate memory B cell responses to antigen rechallenge in vivo.

Follicular helper T cells: lineage and location

Follicular helper T (Tfh) cells are the class of effector T helper cells that regulates the step-wise development of antigen-specific B cell immunity in vivo. Deployment of CXCR5+ Tfh cells to B cell zones of lymphoid tissues and stable cognate interactions with B cells are central to the delivery of antigen-specific Tfh cell function. Here, we review recent advances that have helped to unravel distinctive elements of developmental programming for Tfh cells and unique effector Tfh cell functions focused on antigen-primed B cells. Understanding the regulatory functions of Tfh cells in the germinal center and the subsequent regulation of memory B cell responses to antigen recall represent the frontiers of this research area with the potential to alter fundamentally the design of future vaccines.

Beyond the lymphocyte predominant cell: CD4+CD8+ T-cells in nodular lymphocyte predominant Hodgkin lymphoma

Hodgkin lymphomas are characterised by the presence of rare malignant cells in a background of non-neoplastic inflammatory cells. Flow cytometric analysis of involved tissues is generally not thought to be useful in establishing the diagnosis, because of the small number of neoplastic cells present. However, two recent studies describing a CD4+CD8+ (double-positive) T-cell population in nodular lymphocyte predominant Hodgkin lymphoma (NLPHL) suggest that flow cytometry could play a role in the diagnosis of this Hodgkin lymphoma subtype. In addition, awareness of this unusual T-cell population is important in avoiding a misdiagnosis of a T-cell neoplasm. Although the function of CD4+CD8+ T-cells in NLPHL is not known, studies of phenotypically similar cells in other settings point to a reactive or regulatory role. CD4+CD8+ T-cells have also been identified in the benign entity progressive transformation of germinal centres (PTGC), suggesting a possible relationship between NLPHL and PTGC.

T follicular helper (TFH) cells in normal and dysregulated immune responses

T cell help for antibody production is a fundamental aspect of immune responses. Only recently has a better understanding of the cellular and molecular mechanisms for T cell help emerged. A subset of T cells, termed T follicular helper cells (T(FH) cells), provides a helper function to B cells and represents one of the most numerous and important subsets of effector T cells in lymphoid tissues. T(FH) cells are distinguishable from Th1 and Th2 cells by several criteria, including chemokine receptor expression (CXCR5), location/migration (B cell follicles), and function (B cell help). Central to the function of CD4(+) T cells is IL-21, a "helper" cytokine produced by T(FH) cells that potently stimulates the differentiation of B cells into Ab-forming cells through IL-21R. Consequently, dysregulation of T(FH) cell function, and over- or under-expression of T(FH) cell-associated molecules such as ICOS or IL-21, most likely contributes to the pathogenesis of certain autoimmune diseases or immunodeficiencies.

Germinal-center organization and cellular dynamics

Germinal centers (GCs) are important sites of antibody affinity maturation. In the classical model, the GC dark zone contains large centroblasts that are rapidly proliferating and undergoing somatic hypermutation of their antibody variable-region genes. Centroblasts give rise to smaller nonproliferating centrocytes in the light zone that compete for binding antigen on follicular dendritic cells. Recently, the approach of real-time imaging of GCs by two-photon microscopy of intact lymph nodes has provided new insights into GC dynamics that both support and challenge fundamental aspects of this model. Here we review recent and older findings on cell migration, proliferation, and interaction dynamics in the GC and discuss a model in which dark- and light-zone cells are morphologically similar, proliferation occurs in both zones, and GC B cells compete for T cell help as well as antigen.

Germinal center helper T cells are dual functional regulatory cells with suppressive activity to conventional CD4+ T cells

Germinal center (GC) reaction is a T cell-dependent process in which activated B cells mature to produce high-affinity Abs and differentiate into memory B cells. The GC microenvironment is almost exclusively reserved for the optimal Ag-specific B cell clonal expansion, selection, and maturation, but lack significant conventional CD4(+) T cell responses. The mechanisms that ensure such a focused B cell response in the GC are not known. In this study, we report that human CD4(+)CD57(+) T cells, which are the major helper T cells in GCs, actively suppress the activation of conventional CD4(+) T cells, particularly Th1 cells, via a direct contact-dependent mechanism and soluble mediators. Our findings demonstrate that GC T cells are unique regulatory cells that provide critical help signals for B cell response but suppress conventional effector T cells in the same local environment.

SAP regulation of follicular helper CD4 T cell development and humoral immunity is independent of SLAM and Fyn kinase

Mutations in SH2D1A resulting in lack of SLAM-associated protein (SAP) expression cause the human genetic immunodeficiency X-linked lymphoproliferative disease. A severe block in germinal center development and lack of long-term humoral immunity is one of the most prominent phenotypes of SAP(-) mice. We show, in this study, that the germinal center block is due to an essential requirement for SAP expression in Ag-specific CD4 T cells to develop appropriate follicular helper T cell functions. It is unknown what signaling molecules are involved in regulation of SAP-dependent CD4 T cell help functions. SAP binds to the cytoplasmic tail of SLAM, and we show that SLAM is expressed on resting and activated CD4 T cells, as well as germinal center B cells. In addition, SAP can recruit Fyn kinase to SLAM. We have now examined the role(s) of the SLAM-SAP-Fyn signaling axis in in vivo CD4 T cell function and germinal center development. We observed normal germinal center development, long-lived plasma cell development, and Ab responses in SLAM(-/-) mice after a viral infection (lymphocytic choriomeningitis virus). In a separate series of experiments, we show that SAP is absolutely required in CD4 T cells to drive germinal center development, and that requirement does not depend on SAP-Fyn interactions, because CD4 T cells expressing SAP R78A are capable of supporting normal germinal center development. Therefore, a distinct SAP signaling pathway regulates follicular helper CD4 T cell differentiation, separate from the SLAM-SAP-Fyn signaling pathway regulating Th1/Th2 differentiation.

Hodgkin's lymphoma associated T-cells exhibit a transcription factor profile consistent with distinct lymphoid compartments

BACKGROUND

Hodgkin's lymphoma (HL) is characterised by an ineffective immune response that is predominantly mediated by CD4+ T-cells.

AIMS

To analyse the expression of the key regulatory T-cell transcription factors (TFs) in the T-cells of HL involved tissues in order to assess the nature of the T(H) immune response in HL.

METHODS AND RESULTS

By immunohistochemistry, GATA3 was strongly and T-bet exclusively expressed in a subset of interfollicular lymphocytes in the reactive lymphoid tissues. In classical HL (CHL), which is generally located in the interfollicular zones, a predominance of T-bet+ T-cells and lesser amounts of GATA3+ and c-Maf+ T-cells was found, concordant with the pattern of the normal interfollicular compartment. In reactive lymphoid tissues, c-Maf was observed mostly in T-lymphocytes within the germinal centres (GCs). Nodular lymphocyte predominance type of Hodgkin's lymphoma (NLPHL) and progressively transformed germinal centres cases, showed a majority of c-Maf+ T-cells, consistent with the pattern in normal GCs. NLPHL cases uniformly showed c-Maf+/CD57+ T-cell rosettes around the neoplastic cells; these rosettes were absent in "paragranuloma-type" T-cell/histiocyte rich B-cell lymphoma.

CONCLUSIONS

T-cell TF expression profiles of the reactive T-cells in both subtypes of HL are in accordance with the expression profile observed in the distinct lymphoid compartments.

The chemokine CXCL13 is a key molecule in autoimmune myasthenia gravis

Myasthenia gravis (MG) is associated with ectopic germinal centers in the thymus. Thymectomy and glucocorticoids are the main treatments but they induce operative risks and side effects, respectively. The aim of this study was to propose new therapies more efficient for MG. We hypothesized that molecules dysregulated in MG thymus and normalized by glucocorticoids may play a key role in thymic pathogenesis. Using gene chip analysis, we identified 88 genes complying with these criteria, the most remarkable being the B-cell chemoattractant (CXCL13). Its expression was increased in thymus and sera of glucocorticoid-untreated patients and decreased in response to treatment in correlation with clinical improvement. Normal B cells were actively chemoattracted by thymic extracts from glucocorticoid-untreated patients, an effect inhibited by anti-CXCL13 antibodies. In the thymus, CXCL13 was preferentially produced by epithelial cells and overproduced by epithelial cells from MG patients. Altogether, our results suggest that a high CXCL13 production by epithelial cells could be responsible for germinal center formation in MG thymus. Furthermore, they show that this gene is a main target of corticotherapy. Thus, new therapies targeting CXCL13 could be of interest for MG and other autoimmune diseases characterized by ectopic germinal center formation.

Cytokine gene expression profile distinguishes CD4+/CD57+ T cells of the nodular lymphocyte predominance type of Hodgkin's lymphoma from their tonsillar counterparts

Little is known about the cytokine profile of nodular lymphocyte predominance Hodgkin's lymphoma (NLPHL) and the significance of the characteristic rosetting CD4(+)/CD57(+) T cells. We analysed the T lymphocyte populations isolated from lymph node suspensions from five patients with NLPHL, two with follicular hyperplasia and progressive transformation of germinal centres (PTGC), three with classical Hodgkin's lymphoma (CHL) and five with hyperplasia of the tonsil. We sorted the T cells based on expression of CD3, CD4 and CD57 by flow cytometry and evaluated the cytokine mRNA profiles of the T cells with quantitative RT-PCR. NLPHL cases were as rich in T cells as the CHL cases, but all NLPHL cases had a much higher frequency of CD4(+)/CD57(+) T cells. In contrast to the CD4(+)/CD57(+) T cells from tonsils, IL2 and IL4 mRNAs were consistently absent from the CD4(+)/CD57(+) T cells of NLPHL. Even after stimulation, no IL4 transcripts could be detected in the CD4(+)/CD57(+) T cells of NLPHL. On the other hand, IFNgamma transcripts were elevated in NLPHL and PTGC T cell subsets as compared to tonsillar T cell subsets. IL13 mRNA was exclusively produced by the T cells of CHL cases, indicating that IL13 may be a key cytokine in CHL. In conclusion, elevated levels of CD4(+)/CD57(+) T cells are characteristic of NLPHL and these T cells display a distinct cytokine mRNA profile.

Follicular B helper T cells in antibody responses and autoimmunity

T-cell help for B cells is essential for high-affinity antibody responses and B-cell memory. Recently, the identity of a discrete follicular population of T cells that has a crucial role in this process has become clearer. Similar to primed CD4(+) T cells in the tonsils and memory CD4(+) T cells in the peripheral blood, this follicular population of T cells expresses CXC-chemokine receptor 5 (CXCR5). Owing to their distinct homing preferences and helper function, these T cells differ from T helper 1 and T helper 2 cells and have been denoted follicular B helper T cells. Here, we outline the central role of this subset in normal and pathological immune responses.

Human tonsil implants xenotransplanted in SCID mice display broad lymphocytic diversity and cellular activation profile similar to those in the original lymphoid organ

BACKGROUND

Models consisting of human immune cells in suspension transferred to severe combined immune deficient (SCID) mice have been invaluable for studying immune response, autoimmunity, and lymphomagenesis. The dissemination of human cells within the mouse body hampers immune functionality with time and favorites the development of human graft vs. mouse host (GvH) disease. To circumvent these limitations we surgically implanted tonsil pieces subcutaneously in SCID animals (hu-ton-SCID mice). Recall humoral responses was elicited and animals did not suffer from signs of GvH disease. A detailed cell subset and cell activation analysis of implants has not yet been reported.

METHODS

Implants from 86 hu-ton-SCID mice were evaluated by immunohistochemistry and flow cytometry analyses to assess human lymphoid cell subpopulation surviving with time after implantation, and to evaluate status of human cell activation.

RESULTS

B cells persist over 3 months in implants. The proportion of class and type-specific Ig+ cells varied between implants, but as a whole IgG+ cells were more abundant than IgA+, and IgM+ cells, and kappa+ cells predominated over lambda+ cells. The mean proportions of these cells resemble those in the original tonsil. Fine analysis of CD19+ B cells demonstrated no expansion of activated (CD5+, CD23+, CD69+) B cells in implants compared with tonsils, and a decrease of CD19+CD77+ B cells corresponding to a centroblastic phenotype, which is consistent with the disappearance of follicular structure in implants. Double positive CD20+CD27+ memory B cells were detected in implants by immunohistochemistry. T cell CD4+CD8-/CD4-CD8+ ratios were about 4 in implants, that is similar to those in tonsils, and there was no expansion of CD3+CD4+CD8+ and of CD3+CD4-CD8- T-cell subpopulations. T cells activation markers (CD25, CD69) were similarly expressed in implants and tonsils, and implants contained cells with a memory T cell phenotype (CD45RO). Finally cells within implants depicted a low rate of proliferation when assessed by Ki-67 expression levels.

CONCLUSIONS

Compared with original tonsils, tonsil implants in hu-ton-SCID mice lose the germinal center architecture, which is correlated with the decrease of CD77+ B cells, but conserve T and B cell subpopulation diversity, notably memory cells. In addition, implant T and B cells are not differently activated when compared with those in original tonsils and do not proliferate extensively. These observations indicate indirectly absence of GvH reaction at the cellular level in this model. Collectively, the detailed implant cellular characterization in the hu-ton-SCID model provides a strong rationale for the use of this model in the study of human recall antibody response.

Regulatory T cells can migrate to follicles upon T cell activation and suppress GC-Th cells and GC-Th cell-driven B cell responses

How Tregs migrate to GCs, and whether they regulate the helper activity of the T cells in GCs (GC-Th cells) remains poorly understood. We found a T cell subset in human tonsils that displays potent suppressive activities toward GC-Th cell-dependent B cell responses. These Tregs with the surface phenotype of CD4+CD25+CD69- migrate well to CCL19, a chemokine expressed in the T cell zone, but poorly to CXCL13, a chemokine expressed in the B cell zone. This migration toward the T cell-rich zone rapidly changes to trafficking toward B cell follicles upon T cell activation. This change in chemotactic behavior upon activation of T cells is consistent with their switch in the expression of the 2 chemokine receptors CXCR5 and CCR7. CD4+CD25+CD69- Tregs suppress GC-Th cells and GC-Th cell-induced B cell responses such as Ig production, survival, and expression of activation-induced cytosine deaminase. Our results have identified a subset of Tregs that is physiologically relevant to GC-Th cell-dependent B cell responses and a potential regulation mechanism for the trafficking of these Tregs to GCs.

Human CD57+ germinal center-T cells are the major helpers for GC-B cells and induce class switch recombination

Background

The function of CD57⁺ CD4⁺ T cells, constituting a major subset of germinal center T (GC-Th) cells in human lymphoid tissues, has been unclear. There have been contradictory reports regarding the B cell helping function of CD57⁺ GC-Th cells in production of immunoglobulin (Ig). Furthermore, the cytokine and co-stimulation requirement for their helper activity remains largely unknown. To clarify and gain more insight into their function in helping B cells, we systematically investigated the capacity of human tonsil CD57⁺ GC-Th cells in inducing B cell Ig synthesis.

Results

We demonstrated that CD57⁺ GC-Th cells are highly efficient in helping B cell production of all four subsets of Ig (IgM, IgG, IgA and IgE) compared to other T-helper cells located in germinal centers or interfollicular areas. CD57⁺ GC-Th cells were particularly more efficient than other T cells in helping GC-B cells but not naïve B cells. CD57⁺ GC-Th cells induced the expression of activation-induced cytosine deaminase (AID) and class switch recombination in developing B cells. IgG1-3 and IgA1 were the major Ig isotypes induced by CD57⁺ GC-Th cells. CD40L, but not IL-4, IL-10 and IFN-γ, was critical in CD57⁺ GC-Th cell-driven B cell production of Ig. However, IL-10, when added exogenously, significantly enhanced the helper activity of CD57⁺ GC-Th cells, while TGF-β1 completely and IFN-γ partially suppressed the CD57⁺ GC-Th cell-driven Ig production.

Conclusions

CD57⁺CD4⁺ T cells in the germinal centers of human lymphoid tissues are the major T helper cell subset for GC-B cells in Ig synthesis. Their helper activity is consistent with their capacity to induce AID and class switch recombination, and can be regulated by CD40L, IL-4, IL-10 and TGF-β.

Cutting edge: gamma delta T cells provide help to B cells with altered clonotypes and are capable of inducing Ig gene hypermutation

It has not been resolved whether gammadelta T cells can collaborate with germinal center B cells and support Ig hypermutation during an Ab response to a truly defined T-dependent Ag. In this study, we show that in the absence of alphabeta T cells, immunization with the well-defined T-dependent Ag, (4-hydroxy-3-nitrophenyl) acetyl (NP) conjugate, was able to induce Ig hypermutation. However, the clonotypes of B cells responding to NP were dramatically altered in TCR beta(-/-) mice. Unlike B cells in wild-type mice that use canonical VDJ rearrangements, most NP-responding B cells in mutant mice use analog genes of the J558 gene family. In addition, the majority of anti-NP Abs produced in mutant mice use kappaL chain instead of lambda1L chain, which dominates in mice of Igh(b) background. Thus, the B cell population that collaborates with gammadelta T cells is distinct from B cells interacting with conventional alphabeta Th cells.

Germinal centers regulate human Th2 development

In the present study we demonstrate that all CD4(+) T cells in human tonsil expressing the Th2-selective receptor chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) also 1) express high levels of CXCR5, and 2) display a transitional CD45RA/RO phenotype and consistently do not produce significant amounts of cytokines when immediately analyzed ex vivo. Hence, they represent precursors of Th2 effector cells, a conclusion confirmed by their robust production of IL-4, IL-5, and IL-13, but not IFN-gamma, after in vitro activation. CD4(+) T cells, which express only intermediate levels of CXCR5, instead develop into IFN-gamma-producing cells under identical culture conditions, thus establishing a correlation between relative levels of CXCR5 expression and the acquired cytokine profile. Because CXCR5 is critically involved in follicular localization, the results suggest that these CRTH2(+) Th2 cells preferentially develop their cytokine-producing phenotype within germinal centers (GCs), whereas extrafollicular differentiation instead promotes Th1 development. In support for this proposal, we show that T cells with an intermediate expression of CXCR5 can be forced to also produce IL-4 and IL-13 if cultured with allogenic GC B cells. Finally, we demonstrate that the previously described CD57(+) GC T cells also express high levels of CXCR5 but instead of comprising a Th2 precursor, they represent anergized T cells. Taken together, these data suggest that GCs and B cells regulate CD4(+) T cell differentiation in a finely tuned fashion, either by promoting differentiation of Th2 cells, which apparently leave the lymphoid tissue before evolving a cytokine-producing phenotype, or by furnishing T cell unresponsiveness.

Plasmacytoid dendritic cells: from specific surface markers to specific cellular functions

We have recently described a panel of monoclonal antibodies (mAb), that recognize two novel leukocyte surface antigens, BDCA-2 and BDCA-4. BDCA-2 is a novel type II C-type lectin specifically expressed by plasmacytoid dendritic cells (PDCs) that can internalize antigen for presentation to T cells. Furthermore, signaling via BDCA-2 may play a role in switching from interferon (IFN)-alpha/beta-controlled to interleukin (IL)-12-controlled immune response pathways, as triggering of BDCA-2 potently inhibits secretion of IFN-alpha/beta by PDCs and thereby promotes IL-12 p70 production in PDCs and other cells. Viruses may exploit this switch to escape innate antiviral immunity, but it may be beneficial for patients with systemic lupus erythematosus (SLE) if induced, for instance by anti BDCA-2 mAb treatment. BDCA-4 is shown here to be identical to neuropilin-1 (NP-1), a neuronal receptor for the axon guidance factors belonging to the class-3 semaphorin subfamily, and a receptor on endothelial and tumor cells for vascular endothelial growth factor (VEGF-A). In blood and bone marrow, BDCA-4/NP-1 is exclusively expressed on PDCs, but in tonsils also on a few other cells, primarily follicular B helper memory T cells (T(FH)).

Follicular dendritic cell-mediated up-regulation of CXCR4 expression on CD4 T cells and HIV pathogenesis

Follicular dendritic cells (FDCs) represent a major reservoir of HIV, and active infection occurs surrounding these cells, suggesting that this microenvironment is highly conducive to virus transmission. Because CD4 T cells around FDCs in germinal centers express the HIV coreceptor, CXCR4, whereas CD4 lymphocytes in many other sites do not, it prompted the hypothesis that FDCs may increase CXCR4 expression on CD4 T cells, thereby facilitating infection. To test this, HIV receptor/coreceptor expression was determined on CD4 T cells cultured with or without FDCs, and its consequence to infection was assessed by measuring virus binding and entry. FDCs had little effect on CCR5 or CD4 expression but increased CXCR4 expression on CD4 T cells. FDC-mediated up-regulation of CXCR4 on CD4 T cells occurred by 24 h and was sustained for at least 96 h in vitro, and FDC-CD4 T cell contact was necessary. Importantly, increased CXCR4 expression directly correlated with increased binding and entry of HIV-1 X4 isolates. Furthermore, CD4(+)CD57(+) germinal center T cells expressed high levels of CXCR4 and supported enhanced entry of X4 HIV compared with other CD4 T cells from the same tissue. Thus, in addition to serving as a reservoir of infectious virus, FDCs render surrounding germinal center T cells highly susceptible to infection with X4 isolates of HIV-1.

Dismantling the germinal center: comparing the processes of transformation, regression, and fragmentation of the lymphoid follicle

The pathologic and immunologic features of the formation of the germinal center have been extensively studied. The process of dissolution of the germinal center is not as well understood. Different patterns of germinal-center breakdown are commonly encountered in diagnostic lymph node biopsy specimens and frequently present difficulties in diagnosis. The current immunologic understanding of germinal-center dynamics is reviewed and correlated with the histologic and immunophenotypic features of three broad classes of germinal center dissolution, namely progressive transformation, regression, and follicle fragmentation. The author suggests that these different patterns represent alternate responses to antigenic stimulation. The relationships of progressive transformation to Hodgkin's disease and of follicular regression to Castleman's disease are discussed.

T-cell receptor repertoire in human germinal centres

A search for an antigen-driven expansion of T lymphocytes in the inflamed joints in rheumatoid arthritis (RA) patients have been going on for decades. We here analyzed the human germinal centre T-cell receptor (TCR) Vbeta gene usage with polymerase chain reaction (PCR) combined with sequence analysis, to address the question of clonality in tonsils and synovial tissue from RA patients. Our data show a large degree of TCR heterogeneity in both these histological structures. Furthermore, clonally related T cells were found within different closely located germinal centres indicating either an active T-cell migration between germinal centres (GC) or that a T-cell clone may seed more than one GC.

TH1 cytokine response of CD57+ T-cell subsets in healthy controls and patients with alcoholic liver disease

Patients with chronic inflammatory diseases, including Crohn's disease and rheumatoid arthritis, as well as those with certain viral infections, and patients who are transplant recipients or who have certain hematologic malignancies have been observed to have CD57+ T cell expansion in both CD4+ and CD8+ subsets. We have reported previously that alcoholic patients also have CD57+ T cell expansion. Because many alcoholics become seriously deficient in cell-mediated immunity, it is of interest to determine whether the expanded CD57+ subsets can respond to stimulation with normal T helper cell subtype 1 (TH1) cytokine production. We report evaluation of the CD57 T-cell subsets of patients with alcoholic liver disease (ALD) with the use of cytoplasmic staining after stimulation through the T-cell receptor (TCR). The CD57+ subsets of the T cells of both healthy individuals and patients with ALD express significantly higher amounts of cytoplasmic tumor necrosis factor-alpha (TNF-) and interferon-gamma (IFN-) after 6 h of stimulation than do the CD57- subsets. This increased production can persist up to 46 h of continuous stimulation. Under these assay conditions, very little cytoplasmic interleukin (IL)-4 is observed in the T cells of either healthy control subjects or patients with ALD. Measurement of cytokine secretion by sort-purified CD57 T-cell subsets with the use of enzyme-linked immunosorbent assay (ELISA) shows that the CD57+ T-cell subset produces 18- to 30-fold more TNF- and IFN-, respectively, than does the CD57- subset in the first 12 h of stimulation. This response requires only stimulation through the TCR for the CD57+ subset, whereas significant secretion by the CD57- subset requires added IL-2 or anti-CD28 antibody. These results are consistent with the concept of the CD57+ T-cell subset as a differentiated effector cell and demonstrate that patients with ALD who are not drinking at the time of evaluation have normal or increased immediate TH1 T-cell responses.

Subspecialization of CXCR5+ T cells: B helper activity is focused in a germinal center-localized subset of CXCR5+ T cells

The T helper (Th) cell pool is composed of specialized cells with heterogeneous effector functions. Apart from Th1 and 2 cells, CXCR5+ T cells have been suggested to be another type of effector T cell specialized for B cell help. We show here that CXCR5+ T cells are heterogeneous, and we identify subsets of CXCR5+ CD4 T cells that differ in function and microenvironmental localization in secondary lymphoid tissues. CD57+CXCR5 T cells, hereafter termed germinal center Th (GC-Th) cells, are localized only in GCs, lack CCR7, and are highly responsive to the follicular chemokine B lymphocyte chemoattractant but not to the T cell zone EBI1-ligand chemokine. Importantly, GC-Th cells are much more efficient than CD57-CXCR5+ T cells or CXCR5- T cells in inducing antibody production from B cells. Consistent with their function, GC-Th cells produce elevated levels of interleukin 10 upon stimulation which, with other cytokines and costimulatory molecules, may help confer their B cell helper activity. Our results demonstrate that CXCR5+ T cells are functionally heterogeneous and that the GC-Th cells, a small subset of CXCR5+ T cells, are the key helpers for B cell differentiation and antibody production in lymphoid tissues.

Peripheral T-cell lymphoma with follicular involvement and a CD4+/bcl-6+ phenotype

A truly follicular pattern is thought to be restricted to B-cell lymphomas. We observed a prominent follicular growth pattern in three cases of nodal peripheral T-cell lymphomas, of which two were initially diagnosed as follicular lymphomas. All three patients were male, ranged in age from 50 to 70 years, and had generalized lymphadenopathy at the time of diagnosis. The follicles were sharply demarcated in two cases and large and vague in one case; in all cases, they contained abundant follicular dendritic cells. Neoplastic cells were small to medium, with irregular cleaved or round nuclei and clear cytoplasm, which was abundant in one case. Lymphoma cells in all cases were CD4+ CD8- CD57- bcl-6, with CD10 coexpression in 2 cases. Clonal rearrangement of the gamma chain of the T-cell receptor gene was demonstrated in each case. These cases expand the differential diagnosis of lymphomas with a follicular growth pattern and suggest that neoplastic T cells may have the capacity to induce or home to follicular structures.

Highly restricted spread of HIV-1 and multiply infected cells within splenic germinal centers

The tremendous dynamics of HIV infection finds expression in the tempo of sequence diversification. Genetic diversity calculations require the clearance of a majority of infected cells, the obvious predator being anti-HIV immune responses. Indeed, infiltration of germinal centers (GCs) by HIV-specific CD8(+) cytotoxic T lymphocytes has been described. A corollary to this description would be limited diffusion of virus within lymphoid structures. HIV efficiently infects and replicates mainly in activated CD4(+) T lymphoblasts. These cells are found within GCs after their activation in the adjacent periarteriolar lymphoid sheath (PALS). Here GCs and PALS have been dissected from consecutive 10-micrometer sections through splenic tissue from three HIV-1-infected patients. Nested PCR amplification of the two first hypervariable regions of the env gene indicated that 38-78% of sections contained HIV-infected cells. Since there are several hundred CD4(+) T cells per GC section, approximately 0.09-0.64% harbor proviral DNA. Such a low frequency not only suggests that virions on the follicular dendritic cell surfaces do not readily infect adjacent T cells but also indicates highly restricted spread of HIV within GCs and the PALS. Sections were heavily infiltrated by CD8(+) cells, which, together with a large body of extant data, suggests that the majority of infected cells are destroyed by HIV-specific cytotoxic T lymphocytes before becoming productively infected. Finally, sequence analysis revealed that those HIV-positive cells were multiply infected, which helps explain widespread recombination despite a low overall frequency of infected cells.

Cellular interactions involved in Th cell memory

The cellular interactions involved in maintaining CD4+ T cell memory have hitherto not been identified. In this report, we have investigated the roles played by B cells and dendritic cells (DCs) in this process. We show that long-lasting Th cell memory depends on the presence of B cells, but that direct Ag presentation by B cells is not required. Instead, Ag presentation by DCs is critical for the survival of memory Th cells. DCs presenting specific Ag can be detected in animals long after immunization. These findings support a model in which B cells provide an environment in which Ags may be trapped and retained. This Ag is periodically presented to memory CD4+ T cells by DCs, providing an essential survival signal.

Human tonsillar germinal center T cells are a diverse and widely disseminated population

Germinal center (GC) T cells are a poorly investigated population with a unique helper-inducer memory phenotype. Murine GC T cells have been demonstrated to be oligoclonal and antigen specific. The aim of this study was to investigate the diversity of human tonsillar GC T cells. Immunohistochemistry with a panel of different TCRVbeta family antibodies and sequencing of TCRgamma gene rearrangements obtained from individual microdissected GC demonstrated local diversity of human tonsillar GC T cells. No evidence of local clonal dominance or of a local migratory pathway from the T cell zone to adjacent GC was seen. Primers specific to the junctional region of the TCRgamma gene of individual GC T cell clones were designed, and used to trace the dissemination of the clones. One was found to be concentrated locally. Both clones studied were present in both of the patient's tonsils, suggesting widespread dissemination. In addition, no evidence of somatic hypermutation was found in the TCRgamma gene sequences, or in expressed TCRalpha and beta gene sequences obtained by reverse transcriptase-PCR from isolated tonsillar GC.

Lymphocyte subsets in human tonsils: the effect of age and infection

The aim of the present study was to determine the effect of repeated tonsillitis on the development of lymphocyte subsets in the tonsils and among peripheral blood lymphocytes (PBL) of children. Subsets of T- and B cells were analyzed in the tonsils and in PBL of patients undergoing tonsillectomy for idiopathic tonsillar hypertrophy, recurrent tonsillitis, or tonsillar hypertrophy and tonsillitis. The majority of the CD4+ cells in the tonsils displayed the CD45RO+ phenotype, while the majority of those in the PBL displayed the CD45RA+ phenotype. Likewise, the proportion of CD45RO+CD8+ cells was higher in the tonsils than among PBL. The proportion of CD4 cells expressing the CD45RO marker increased with age among PBL, but not in the tonsils. B cells, detected by their CD19, CD20, and CD21 markers, were three times more abundant in the tonsils than in the PBL. The proportion of CD38+ cells showed a negative correlation with age, both in the tonsils and among PBL. Among PBL a striking age-related reduction was seen in the proportion of CD19+, CD21+ and CD38+CD21+ B cells. In contrast, in the tonsils age-related changes could be detected only in the proportion of CD21+CD38+ cells. No difference among patients with various clinical diagnoses was detectable in any of the T- and B cell subsets in the tonsils and PBL. Thus, lymphocyte subsets evolve independently in the tonsils and peripheral blood, with the repeated antigenic challenge of tonsillar lymphocytes not influencing circulating memory cells.

B cell-driven HIV type 1 expression in T cells: an essential role of CD86 costimulatory molecule

During HIV-1 infection, HIV-1 is sequestered and actively replicates within lymphoid organs, mainly in areas essential for antigen-specific T-B interactions. We investigated whether cognate T-B interactions not only drive humoral response to HIV-1 but also enhance viral replication. Costimulation of in vitro HIV-1-infected tonsillar T cells with autologous or allogeneic activated B cells increased both viral replication and T cell proliferation. Addition of CD86 MAb to cocultures inhibited most p24 (84 +/- 12%, n = 13) and IL-2 (99 +/- 2%, n = 6) production, decreased T cell proliferation by 46 +/- 15% (n = 13), and decreased TNF-alpha and IFN-gamma production by 67 +/- 17% (n = 6) and 53 +/- 6% (n = 6), respectively. In contrast, CD80 MAb, which strongly inhibited IL-2 production (77 +/- 10%, n = 6), moderately downregulated p24 and TNF-alpha production (29 +/- 21%, n = 13 and 34 +/- 10%, n = 6, respectively) and did not decrease T cell proliferation (8 +/- 10%, n = 13) or IFN-gamma production (14 +/- 13%, n = 6). We thus showed that B cells deliver a potent CD86/CD28 costimulatory signal that induces T cell proliferation and simultaneously enhances HIV-1 replication. CD86+ B cells, mainly localized within the light zone of germinal centers, might thus favor active in situ replication of HIV-1 in response to each new challenge by T-dependent antigens.

Outer periarteriolar lymphoid sheath arrest and subsequent differentiation of both naive and tolerant immunoglobulin transgenic B cells is determined by B cell receptor occupancy

T-dependent B cell responses in the spleen are initiated in the outer periarteriolar lymphoid sheath (PALS) and culminate in the generation of proliferative foci and germinal center reactions. By pulsing anti-hen egg lysozyme (HEL) immunoglobulin transgenic (IgTg) B cells with various concentrations of HEL in vitro before adoptive transfer into normal recipients, it was shown that a critical number of B cell receptors (BCRs) must be ligated for B cells to undergo arrest in the outer PALS. T cell help was manipulated independently of the BCR stimulus by incubating B cells expressing the appropriate major histocompatibility complex class II antigen with a peptide recognized by CD4(+) TCR Tg T cells. B cells which either failed to arrest in the outer PALS due to a subthreshold BCR stimulus, or arrested only transiently due to the brevity of the BCR stimulus, underwent an abortive response within the follicles when provided with T cell help. In contrast, naive B cells stimulated by a sustained, suprathreshold concentration of either foreign or self-antigen and given T cell help, proliferated in the outer PALS and then differentiated. Outer PALS arrest was not influenced by the nature of the B cells occupying the follicle, but appeared to be determined solely by the magnitude of BCR stimulation. Thus antigen-pulsed B cells arrested in the outer PALS in an identical manner irrespective of whether the follicles comprised a population of normal B cells with multiple specificities, a monoclonal naive population, or a monoclonal population of tolerant B cells. In addition, tolerant B cells were found to relocate from the follicles to the outer PALS of HEL/anti-HEL double Tg mice in which the concentration of soluble self-antigen had been increased by zinc feeding. Similarly, when anti-HEL Tg mice were crossed with a second HEL Tg strain expressing a higher concentration of soluble HEL, the tolerant anti-HEL Tg B cells were located constitutively in the outer PALS. Thus, subtle variations in antigen concentration resulted in dramatic changes in positioning of B cells within the spleen. A series of mixed bone marrow chimeras in which the effective antigen concentration was inversely related to the number of self-reactive B cells due to absorption of antigen by transgene-encoded membrane and secreted Ig, was used to confirm that alteration in B cell position previously attributed to changes in follicular composition could be explained on the basis of available antigen concentration, rather than the diversity of the repertoire.

Massive production of Th2 cytokines by human CD4+ effector T cells transiently expressing the natural killer cell marker CD57/HNK1

We have reported previously that uncommitted human CD4+ CD45RO- T cells default to the T-helper type 1 (Th1) pathway, if they are costimulated by anti-CD3 plus anti-CD28 monoclonal antibodies (mAb). In contrast, 5% of the uncommitted T cells differentiate into Th2 cells, if they are stimulated by anti-CD28 plus interleukin-2 (IL-2) in the absence of T-cell receptor (TCR) signals. The anti-CD28/IL-2-induced proliferation (and the resulting Th2 commitment) was not affected by neutralizing anti-IL-4 mAb, suggesting a non-conventional IL-4-independent Th2 differentiation pathway. Here we report that the respective CD4+ Th2 cells (but not the Th1 cells) coexpressed the natural killer (NK) cell marker HNK1/CD57. Expression of CD57 on Th2 cells required CD28 stimulation, and was suppressed by CD3/TCR signals. However, Th2 effector cells displayed a TCR V beta-chain usage comparable to that of committed Th1 cells (with V beta 8 dominating). Our data suggest that expression of CD57 on human CD4 T cells may be associated with defined stages of Th2 cell activation/differentiation, and may not necessarily characterize a separate T-cell lineage. The induction of cytokine production and B-cell helper function in both Th1 and Th2 populations required CD3/TCR signalling in costimulation with anti-CD28 or IL-2. Importantly, anti-CD28/IL-2-primed Th2 cells readily secreted IL-4 and induced IgE production by surface IgE- B cells in response to the first TCR signal and independent of previous contact with IL-4. Therefore, CD4+ CD57+ T cells responded comparably to murine CD4+ NK1.1+ T cells, which are critical for the development of Th2/IgE immune responses in vivo. The possible role of human CD4+ CD57/HNK1+ Th2-like cells in cancer, infection and allergy is discussed.

Germinal centre CD4+ T cells are an important site of HIV replication in vivo

OBJECTIVE

CD4+ T cells are the main target for HIV. However, the highest HIV antigen concentration in infected subjects accumulates on the cell surface of follicular dendritic cells in the germinal centres of the lymphoid tissue. Germinal centres contain a T-helper cell subset which expresses CD57 molecules. Here we analysed virus replication and viral load in CD57+CD4+ germinal centre T cells and in the CD4+ T cells found mostly outside germinal centres (CD57-CD4+).

METHODS

Peripheral blood mononuclear cells and lymph-node cells were prepared, stained for CD4 and CD57 and purified by FACS. Defined cell numbers of CD4+CD57+ cells and CD4+CD57- cells were sorted directly into polymerase chain reaction (PCR) tubes by FACS, equipped with an automated cell deposition unit and analysed by PCR to detect proviral DNA. Based on Poisson distribution, the expected level of infection was calculated. Viral replication was determined by amplifying double-spliced, single-spliced, and full-length transcripts of HIV using serially diluted cDNA of the FACS-sorted cells.

RESULTS

An up to 10-fold higher frequency of infected cells was found in the CD57+CD4+ germinal centre T cells compared with CD57-CD4+ T cells. Furthermore, active viral replication was detected almost exclusively in the CD57+CD4+ T cells.

CONCLUSIONS

The CD57+CD4+ germinal centre T cells are one of the sites of HIV infection and replication that may play a pivotal role in the pathogenesis of HIV infection.

Terminal differentiation of human germinal center B cells in vitro

In order to define an in vitro culture system allowing growth of single human germinal center B cells (GC-B), we have studied the proliferation and differentiation of human tonsillar GC-B, and subsets thereof, when cultured together with murine EL-4 thymoma cells in the "EL-4 system." The cells were analyzed and compared to resting tonsillar B cells with respect to phenotypic changes, proliferation, Ig secretion, intracellular Ig levels, and growth abilities under limiting dilution conditions. It was found that GC-B differentiated terminally to Ig-secreting cells with the phenotypic features of plasma cells in a similar manner to tonsillar resting B cells. The GC-B proliferated for 4-5 days, followed by a loss of GC-B phenotype and an increase in intracellular immunoglobulin levels. Over a 10-day culture period a larger proportion of the Ig produced by GC-B was IgG and IgA, as compared to resting B cells, indicating that these cells switched isotype more easily or had already switched in the germinal center prior to the culture period. Analysis of frequencies of Ig-producing cells revealed that 1/3.8 of GC-B and less than 1/10 of the centroblast B cell subpopulation (CB-B) differentiated toward Ig-producing cells when cultured in the EL-4 system whereas 1/1.25 and 1/1.5 of peripheral blood B cells (PBL-B) and resting tonsillar B cells did so, respectively. Taken together, these findings show that tonsillar GC-B differentiate in a similar manner to resting B cells when cultured in the EL-4 system, and we conclude that these conditions allow manipulation of GC-B in single cell cultures in vitro.

Nature of the T lymphocytes in lymphocyte predominance Hodgkin's disease

Little is known about the function of the T lymphocytes in lymphocyte predominance Hodgkin's disease. We report here the case of a 37-year-old man with a diffuse LPHD, featuring a similar increase in T lymphocytes in both the peripheral blood and the tumor, thus allowing for their characterization by functional assays. These cells were CD4+CD45RO+ and produced high amounts of IL-2 and IFN-gamma, consistent with a TH1-type profile. This subset of T helper cells is involved in cellular immunity and could reflect a cytotoxic reaction directed against the neoplastic cells.

Gamma delta T-cell help in responses to pathogens and in the development of systemic autoimmunity

Mice rendered deficient in alpha beta T-cells by single-gene knockout mutation show enhanced levels of autoantibody formation and even some symptoms of autoimmune disease. This is remarkable given that most experimental studies heretofore have indicated that the development of autoimmune disease is highly multigenic, requiring the complementary actions of multiple loci. The basis of the phenomenon in alpha beta T-cell-deficient mice appears to be the provision of help to B-cells by other cells, including gamma delta T-cells. Perhaps surprisingly, gamma delta T-cell help seems quite efficacious, particularly after infection, when it can culminate in the formation of germinal centers. Furthermore, two independent sets of studies reviewed here indicate that significant levels of self-reactive IgG can also be provoked by gamma delta T-cells independent of germinal center formation. The task ahead is to integrate this pathway into the physiologic immune responses to healthy individuals, immunocompromised individuals, and newborns.

Gamma delta T cell help of B cells is induced by repeated parasitic infection, in the absence of other T cells

BACKGROUND

gamma delta T cells, like alpha beta T cells, are components of all well-studied vertebrate immune systems. Yet, the contribution of gamma delta T cells to immune responses is poorly characterized. In particular, it has not been resolved whether gamma delta cells, independent of any other T cells, can help B cells produce immunoglobulin and form germinal centers, anatomical foci of specialized T cell-B cell collaboration.

RESULTS

TCR beta-/- mice, which lack all T cells except gamma delta T cells, routinely displayed higher levels of antibody than fully T cell-deficient mice. Repeated parasitic infection of TCR beta-/- mice, but not of T cell-deficient mice, increased antibody levels and induced germinal centers that contained B cells and monoclonal gamma delta cells in close juxtaposition. However, antibody specificities were more commonly against self than against the challenging pathogen. gamma delta T cell-B cell help was not induced by repeated inoculation of TCR beta-/- mice with mycobacterial antigens.

CONCLUSIONS

In the absence of any other T cells, gamma delta T cell-B cell collaboration can be significantly enhanced by repeated infection. However, the lack of obvious enrichment for antibodies against the challenging pathogen distinguishes gamma delta T cell help from alpha beta T cell help induced under analogous circumstances. The increased production of generalized antibodies may be particularly relevant to the development of autoimmunity, which commonly occurs in patients suffering from alpha beta T cell deficiencies, such as AIDS.

T helper cells in murine germinal centers are antigen-specific emigrants that downregulate Thy-1

After immunization, activated splenic T cells proliferate in periarteriolar lymphoid sheaths (PALS) and subsequently migrate to the lymphoid follicle where they enter nascent germinal centers. Analysis of TCR V(D)J gene rearrangements indicates extensive emigration, frequently involving more than a single white pulp region. These migrants constitute a unique set of T helper cells that express antigen-specific alpha beta TCR, CD3, and CD4, but little or no Thy-1, a differentiation antigen present on the great majority of peripheral murine T lymphocytes. The origin of CD4+ Thy-1 follicular T cells appears to be the Thy+ population in the PALS, as both sets commonly share identical V(D)J rearrangements.

Immunoglobulin switch transcript production in vivo related to the site and time of antigen-specific B cell activation

Immunoglobulin (Ig) class switch recombination is associated with the production and splicing of germline IgCH messenger RNA transcripts. Levels of gamma 1 transcripts in mouse spleen sections were assessed by semiquantitative analysis of reverse transcriptase polymerase chain reaction (PCR) products during primary and secondary antibody responses to chicken gamma globulin (CGG). This was correlated with the appearance of CGG-specific B cells and their growth and differentiation to plasma cells. After primary immunization with CGG, gamma 1 switch transcripts appeared after 4 d, peaked at a median of six times starting levels between 10 and 18 d after immunization, and returned to background levels before secondary immunization at 5 wk. By contrast, after secondary challenge with CGG, a sevenfold increase in transcripts occurs during the first d. The level again doubles by day 3, when it is six times that which is seen at the peak of the primary response. After day 4, there was a gradual decline over the next 2-3 wk. Within 12 h of secondary immunization, antigen-specific memory B cells appeared in the outer I zone and by 24 h entered S phase, presumably as a result of cognate interaction with primed T cells. Over the next few hours, they migrated to the edge of the red pulp, where they grew exponentially until the fourth day, when they synchronously differentiated to become plasma cells. The same pattern was seen for the migration, growth, and differentiation of virgin hapten-specific B cells when CGG-primed mice were challenged with hapten protein. The continued production of transcripts after day 3 indicates that switching also occurs in germinal centers, but in a relatively small proportion of their B cells. The impressive early production of switch transcripts during T cell-dependent antibody responses occurs in cells that are about to undergo massive clonal expansion. It is argued that Ig class switching at this time, which is associated with cognate T cell-B cell interaction in the T zone, has a major impact on the class and subclasses of Ig produced during the response.

Follicular dendritic cells and germinal centers

Follicular dendritic cells (FDCs) are stromal cells unique to primary and secondary lymphoid follicles. Recirculating resting B cells migrate through the FDC networks, whereas antigen-activated B cells undergo clonal expansion within the FDC networks in a T cell-dependent fashion, thereby generating germinal centers. Here, B cells undergo somatic mutation, positive and negative selection, isotype switching and differentiation into high-affinity plasma cells and memory B cells. Since the discovery of FDCs by electron microscopy as long-term antigen-retaining cells 30 years ago isolation of FDCs and generation of FDC-like cells lines and of FDC-specific monoclonal antibodies have been achieved. FDCs express all three types of complement receptors as well as Ig-Fc receptors, through which antigen-antibody immune complexes are retained. However, the mechanism that prevents FDCs from internalizing the antigens and retaining them in native form for long periods of time remains obscure. Substantial evidence derived from cultures in vitro indicates that FDCs contribute directly to the survival and activation of peripheral B cells. The adhesion between FDCs and B cells is mediated by ICAM-1 (CD54)-LFA-1(CD11a) and VCAM-VLA-4. T cells may interact with FDCs in a CD40/CD40-ligand-dependent fashion. Whether FDCs originate from hematopoietic progenitors or from stromal elements is still a controversy. New evidence suggests the presence of two types of dendritic cells within human germinal centers: (i) the classic FDCs that express DRC-1, KiM4, and 7D6 antigens represent stromal cells; and (ii) the newly identified CD3-CD4-CD11c- germinal center dendritic cells (GCDC) represent hematopoietic cells that may be analogous to the antigen-transporting cells described in mice. Finally, FDCs appear to be involved in the growth of follicular lymphomas and in the pathogenesis of HIV infection.

Role of apoptosis in HIV disease pathogenesis

After approximately one and a half decades of intensive studies, the exact mechanisms to explain HIV-mediated cytopathicity are still enigmatic and need closer scrutiny. There has been a dichotomy between virological and immunological viewpoints in understanding HIV-mediated cytopathicity, the former emphasizing a killing of infected cells by HIV-1 and the latter emphasizing indirect mechanisms wherein HIV or its soluble component(s) alter CD4 T-cell function and induce susceptibility to apoptosis. Accumulating evidence points to the notion that apoptosis might be a major contributor to the depletion of CD4 T-cells in HIV infection. This review summarizes current information about the regulatory mechanisms of T-cell apoptosis and the role of apoptosis in HIV pathogenesis with the goal of providing an integrated view of HIV cytopathicity.

A subset of CD4+ memory T cells contains preformed CD40 ligand that is rapidly but transiently expressed on their surface after activation through the T cell receptor complex

Signaling through surface CD40 is essential for selecting B cells that have mutated their immunoglobulin variable region genes in germinal centers and is an important signal in the early stages of antibody responses to T cell-dependent antigens. It is shown that a subset of CD45RO+, CD4+ T cells isolated from human tonsil contains preformed 30-35-kD ligand for CD40. This is expressed on their surfaces within 5 min of their antigen-receptor complexes interacting with CD3 epsilon antibodies bound to ox erythrocytes. This surface expression does not require de novo protein synthesis and lasts for only 1-2 h. Preformed CD40 ligand (CD40L) was not detected in any CD4+ CD45RA+ T cells, but > 90% of all CD4+ T cells from the tonsil can be induced to express large amounts of CD40L on culture with phorbol myristate acetate and the calcium ionophore ionomycin. This expression of CD40L starts between 1 and 2 h, peaks at 6 h, and remains at a high level for > 20 h. It is totally prevented by adding a concentration of cycloheximide that inhibits CD25 synthesis by these activated cells. While CD3 epsilon antibody bound to ox red cells is a good inducer of surface expression of CD40L, it is a much less potent inducer of CD40L synthesis than phorbol myristate acetate with ionomycin. Immunohistological analysis of tonsil sections shows that cells containing CD40L are located mainly in the outer zone of germinal centers and the margins of the T zones that are rich in dendritic cells (interdigitating cells). The distribution of these cells is consistent with: (a) their interaction in T zones with B cells that have taken up and processed antigen and (b) their involvement in B cell selection in germinal centers.