The role of ICOS and other costimulatory molecules in allergy and asthma

Activation and differentiation of T cells play a critical role in the pathogenesis of allergies and asthma. Upon encounter with specific antigen, naïve T helper precursor (ThP) cells become activated, an event that is regulated not only by engagement of the T cell receptor (TCR) with peptide presented in the context of MHC class II molecules, but also by a number of costimulatory signals. CD28 engagement by B7-1 and B7-2 on resting ThP cells provides a critical signal for initial cell cycle progression, interleukin-2 production and clonal expansion. However, in recent years, other related members of the immunoglobulin (Ig) family, such as inducible costimulatory molecules (ICOS) and the TNF receptor family members which include OX40, have also been demonstrated to play an important role in providing unique and complementary signals that regulate the outcome of immune responses. These positive costimulatory signals are counterbalanced by signals that dampen down immune responses and include CTLA-4, PD-1 and the recently described Ig superfamily members BTLA and TIM-3. This review discusses the role of these costimulatory signals and their potential involvement in the pathogenesis of asthma and allergic responses.     

Co-stimulation of antigen-specific CD4 T cells by 4-1BB ligand

4-1BB is a member of the TNF receptor family predominantly expressed on activated T cells, and binds an inducible ligand found on B cells, macrophages and dendritic cells. Whereas ligation of 4-1BB has been shown to enhance response of purified CD8 T cells to mitogens, and to augment NK activity and generation of cytotoxic T lymphocytes in vivo, there are little direct data on 4-1BB action during CD4 responses. Using pigeon cytochrome c-presenting fibroblast antigen-presenting cells transfected with 4-1BB ligand (4-1BBL), we show that engaging 4-1BB on naive CD4 cells promotes proliferation, cell cycle progression and IL-2 secretion, and suppresses cell death, all to a similar extent as B7-1 engagement of CD28. In addition, 4-1BBL synergizes with B7 and ICAM to enhance naive CD4 proliferation when antigen is limiting. 4-1BBL alone, and to a greater extent with B7, also augmented IL-2 secretion resting antigen-experienced CD4 cells, as typified by T helper clones, whereas short-term effector cells showed similar levels of proliferation and cytokine secretion regardless of whether 4-1BB was engaged. A major role in augmenting IFN-gamma, IL-4 or IL-5 was not demonstrated. Blocking studies with activated B cells presenting antigen showed that 4-1BB participates in promoting IL-2 production by resting CD4 cells, confirming that 4-1BBL can play a role in antigen-specific CD4 T cell responses.     

In situ expression of B7 and CD40 costimulatory molecules by normal human lung macrophages and epithelioid cells in tuberculoid granulomas.

Normal alveolar macrophages (AM) are not efficient in inducing the proliferation of resting T lymphocytes, and, rather, tend to inhibit pulmonary immune responses. In contrast, epithelioid cells (EC), activated macrophages that play an essential role in the course of granulomatous responses, appear to stimulate T cell proliferation efficiently. The inability of macrophages to deliver potent costimulatory signals through the B7/CD28 and CD40/CD40L pathways could explain their weak accessory cell activity. Using MoAbs and immunohistochemical techniques, however, we found that essentially all AM in normal human lung tissue expressed B7-1, B7-2 and CD40 molecules, and most of these cells were strongly positive. Pulmonary macrophages in other compartments also expressed these costimulatory molecules; no differences in expression were observed comparing macrophages from smokers and non-smokers. Most AM recovered by bronchoalveolar lavage from normal lung segments also strongly expressed B7-1, B7-2 and CD40 molecules. In comparison, resting blood monocytes were B7-1- and only moderately positive for B7-2. Activation of monocytes with lipopolysaccharide (LPS) induced expression of these costimulatory molecules to levels similar to that of AM from the control subjects. EC in granulomatous lesions also expressed easily detectable levels of B7-1, B7-2 and CD40. T lymphocytes within and surrounding the granulomas expressed CD28, the counter-receptor for B7, and many of these T cells also expressed B7-1 and B7-2. These findings suggest that both AM and EC can deliver costimulatory signals through B7-1, B7-2 and CD40 molecules, and indicate that the impairment in accessory cell activity observed for normal AM cannot be attributed to the absence of expression of these costimulatory molecules.     

Reciprocal expression of co-stimulatory molecules,B7-1 and B7-2, on murine T cells following activation

The co-stimulatory B7 molecules (B7-1 and B7-2) are expressed on professional antigen-presenting cells in mice. In this study, we demonstrate that B7-1 (CD80) and B7-2 (CD86) are also expressed on murine T cells in the absence of major histocompatibility complex class II molecules. The temporal expression of these two molecules on T cells varies with the state of activation where resting T cells express B7-2 but show little or no expression of B7-1. Following activation, B7-2 expression is down-regulated and there is a concomitant increase in the expression of B7-1 on the cell surface which peaks at about 72 h. Thus these two co-stimulatory molecules are reciprocally expressed on the T cell surface. This pattern of expression of B7-1 and B7-2 on T cells suggests that these two molecules may have different roles in the generation and regulation of immune responses.     

B7-1 and B7-2: similar costimulatory ligands with different biochemical, oligomeric and signaling properties

B7-1 and B7-2 are homologous costimulatory ligands expressed on the surface of antigen presenting cells (APCs). Binding of these molecules to the T cell costimulatory receptors, CD28 and CTLA-4, is essential for the activation and regulation of T cell immunity. Despite strong structural similarities, B7-1 and B7-2 exhibit different biochemical features, and their binding to the costimulatory receptors results in distinct T cell functional outcomes. Using photobleaching based fluorescence resonance energy transfer (FRET), our previous studies have demonstrated that B7-1 and B7-2 have different cell surface oligomeric states. While B7-1 is present as a dimer, B7-2 exists as a monomer on the cell surface suggesting that the unique cell surface oligomeric states of the costimulatory ligands may play a key role in the regulation of T cell responses. Moreover, signaling via B7-1 and B7-2 in dendritic cells has been reported to be dependent on their simultaneous expression, raising the possibility that their direct interaction or their involvement in synergistic signaling pathways may play a role in the function of antigen presenting cells. We discuss physiological relevance of distinct oligomeric states of B7-1 and B7-2 and address whether these molecules can associate with one another on the cell surface to form hetero-oligomers. Our findings suggest that B7-1 and B7-2 do not form hetero-oligomers, underscoring the biological relevance of dimeric and monomeric state of B7-1 and B7-2, respectively.     

Regulation of CD80/B7-1 and CD86/B7-2 molecule expression in human primary acute myeloid leukemia and their role in allogenic immune recognition

Clinical data and animal models afford evidence for anti-leukemia immunity in humans, but the interactions critical for blast cell recognition are unresolved. Expression of B7 molecules by antigen-presenting cells (APC) provides co-stimulatory signals to T lymphocytes via CD28 and CTLA-4 which prevent the induction of alloantigen-specific tolerance. Conversely, expression of CD40 ligand by stimulated T cells activates APC via CD40. In human hematological B cell malignancies (follicular lymphoma and chronic lymphocytic leukemia), the defect in alloantigen presentation of tumoral cells can be repaired by up-regulation of B7 and other co-stimulatory molecules via CD40. We studied the role of B7 molecules in alloimmune recognition and the various ways to improve the antitumoral response on peripheral blood leukemic cells from 20 patients with a diagnosis of primary acute myeloid leukemia (AML). We focused on myelo/monocytic M4/M5 French-American-British classification subtypes which are considered as the neoplastic counterpart of normal monocytes, a prototypic APC. In one-way mixed lymphocyte reaction of CD4+ T cells against leukemic cells, differences in B7-1, B7-2 or CD40 expression by AML cells did not induce specific cytokine secretion; interleukin (IL)-2 and interferon (IFN)-γ were detected but not IL-4, corresponding to a Th1 pattern. Blockade experiments showed that proliferation and IFN-γ secretion only partially depended on B7 molecules, which in contrast had a pivotal role in IL-2 synthesis. In contrast with murine models which suggest a pivotal role for CD80/B7-1 in the immune response against AML, our data support a greater role for CD86/B7-2, in line with the baseline expression of CD86/B7-2 and lack of CD80/B7-1 on most M4/M5 AML cells. AML cell stimulation via CD40: (1) significantly improved IL-2 secretion but not proliferation of responding T lymphocytes, (2) increased CD54/ICAM-1 expression in three quarters of cases, (3) failed in most cases to induce CD40-specific CD80/B7-1 up-regulation and (4) had a weak effect on CD86/B7-2 expression. These data contrast with the very efficient up-regulation of both B7 co-stimulatory molecule expression and tumoral cell alloimmune recognition following CD40 stimulation in B cell malignancy models. The role of the defective B7 molecule up-regulation by the CD40 pathway in inefficient tumor immunogenicity of primary AML cells has to be further investigated, in particular using transfection experiments of CD80/B7-1-deficient AML cell lines. From our in vitro data we conclude that B7 molecules play an important role in the alloimmune surveillance of AML as suggested by the high B7 molecule dependency of IL-2 secretion. Nonetheless, the contribution of B7 molecules to alloimmune T cell proliferation against primary AML cells in human and the way to improve it – regulation via CD40 in particular – differ from B cell malignancies and murine models, suggesting the requirement for specific strategies in the development of antitumor immunity.     

Cryptococcus neoformans differently regulates B7-1 (CD80) and B7-2 (CD86) expression on human monocytes

To induce a specific response in primary resting T cells, two signals must be provided by antigen-presenting cells (APC). The first antigen-specific signal is mediated by formation of the T cell receptor major histocompatibility complex molecule ternary complexes. The second signal is delivered by interaction of either B7-1 or B7-2 expressed by APC with CD28 or CTLA-4 on T cells. In this study, we examined the modulation of B7-1 and B7-2 molecules on human monocytes exposed to encapsulated or acapsular Cryptococcus neoformans or Candida albicans. In our experimental system, C. albicans or acapsular C. neoformans are able to induce B7-1 expression while the encapsulated yeast is a poor stimulator. A modest increase of B7-2 expression was also observed after monocyte treatment with acapsular C. neoformans or C. albicans, while the encapsulated yeast was ineffective in inducing B7-2 molecules. Kinetic analysis showed the maximum expression of B7-1 after 24 to 48 h. Addition of the opsonic IgG1 mAb 2H1 to monocytes and C. neoformans significantly increased B7-1, but not B7-2, expression. The contribution of B7-1 and B7-2 co-stimulatory (CS) molecules to cryptococcal-specific T cell activation was analyzed and a substantial inhibition of T cell proliferation was observed. In this study we provide the first demonstration of fungal interference in the regulation of CS molecules. Our results suggest a potential mechanism for poor inflammatory responses observed in C. neoformans infections.     

Novel roles for murine complement receptors type 1 and 2 I. Regulation of B cell survival and proliferation by CR1/2

Innate components of the immune system, such as complement are known to have a modulatory effect on adaptive immune responses. Complement receptors are expressed by both B and T lymphocytes and play part in antigen presentation and cellular activation and adhesion events. On murine B cells type 1 and 2 complement receptors (CR1/2) are expressed and form a co-receptor complex together with CD19 and CD81. We used CR1/2 specific antibodies to assess the role these receptors might play in regulating cell cycling events of B cells. We show that a CR1/2 specific antibody fragment, 7G6 scFv can induce the proliferation of mature B cells. This effect is countermodulated by FcR crosslinkage and enhanced by BCR engagement. The proliferative effect is severely impaired in Cr2-/- animals, strengthening the involvement of CR1/2. Transitional B cells are prone to apoptotic death by selection events, yet they are rescued from apoptosis by CR1/2 crosslinkage. CR1/2 ligation by 7G6 scFv alone can induce nuclear translocation of NF-kappaB, supporting the above observations.We conclude that engagement of complement receptor 2 of B cells promotes the survival of both mature and transitional B cells. This activity supplements the previously described adjuvant effects of complement.     

Murine keratocytes function as antigen-presenting cells.

Keratocytes express MHC class I molecules constitutively, and keratocytes stimulated with IFN-gamma express MHC class II molecules. Unstimulated keratocytes constitutively express B7-1 and ICAM-1, as well as low levels of CD40 and 4-1BBL. These findings indicate that keratocytes may deliver both antigen-specific and costimulatory signals to CD4(+) and CD8(+) T cells. To demonstrate that keratocytes expressing B7-1 provide a costimulatory signal to T cells, CD4(+) or CD8(+) mouse T cells were incubated with anti-CD3 mAb and irradiated keratocytes. Enhanced proliferation of both CD4(+) and CD8(+) T cells occurred, and could be inhibited by anti-B7-1 mAb, indicating T cell costimulatory activity by B7-1 on the keratocytes. To demonstrate that keratocytes can deliver an antigen-specific signal, CD4(+) and CD8(+) T cells from herpes-infected mice were incubated with HSV-1-infected, irradiated keratocytes. The resulting T cell proliferation and production of Th1 cytokines (IL-2, IFN-gamma) indicated T cell activation by antigens presented by the infected keratocytes. These results show that keratocytes in the corneal stroma of the mouse can function as antigen-presenting cells and, thus, may play a role in immune-mediated stromal inflammation such as herpetic stromal keratitis.     

B7-1-HSA (CD80-CD24), a recombinant hybrid costimulatory molecule retains ligand binding and costimulatory functions

Optimal activation of na?ve T lymphocyte requires two signals; an antigen-specific signal initiated by engagement of TCR with the antigen-MHC complex and a costimulatory signal independent of the antigen receptor complex. Without the costimulatory signal, T cells become anergic. Various adhesion molecules, such as B7-1 (CD80) and heat stable antigen (HSA, CD24), expressed on antigen presenting cells have been demonstrated to provide costimulatory signals to T cells. It was reported that the combinations of different adhesion molecules could induce even stronger immune response. In this study, we made a hybrid costimulatory molecule, B7-1-HSA, and tested its T cell stimulatory function. Chinese hamster ovary (CHO) cells expressing this hybrid molecule bound both anti-CD80 and anti-CD24 monoclonal antibodies, and induced stronger T cell proliferation than CHO cells expressing B7-1 or HSA alone. These results suggest that the B7-1-HSA hybrid molecule can deliver two costimulatory signals simultaneously that can synergize in inducing T cell proliferation. The purified B7-1-HSA protein reacted with both anti-B7-1 and anti-HSA mAbs in Western blotting and specifically mediated adhesion of Jurkat cells. Furthermore, purified B7-1-HSA molecule spontaneously incorporated onto cell membrane through its glycolipid anchor suggesting that this hybrid costimulatory molecule can be used in protein transfer to develop effective cancer vaccines.     

Regulation of B Cell:T Cell Interactions: Potential Involvement of an Endogenous B Cell Sialidase

In light of the ability of B cells treated with neuraminidase to interact more effectively with T cells, the increased capacity of activated, but not small resting B cells, to interact with T cells could be associated with the level of sialylation on certain B cell surface molecules which influences the effectiveness of the physical interaction between B and T cells. The purpose of this study was to determine if activation of B cells altered sialylation via an endogenous sialidase which affected both the initial interaction between T and B cells and subsequent B cell-induced T cell proliferation. The competitive neuraminidase inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (NeuAc2en), inhibited LPS-mediated enhancement of B cell conjugate formation with Ia-specific T cell clones as well as enhancement of their capacity to stimulate a mixed lymphocyte reaction. The addition of NeuAc2en during LPS stimulation did not affect the surface expression of Ia, LFA-1, ICAM-1 or mB7, suggesting that inhibition of LPS-mediated enhancement by the sialidase inhibitor was not due to changes in the level of expression of the major B cell adhesion or co-stimulatory molecules. Short term stimulation with phorbol myristate acetate (PMA) and ionomycin also enhanced the ability of resting B cells to form antigen specific T:B conjugates. However, activation of B cells with PMA and ionomycin or with LPS did not change the capacity of a sialic acid specific lectin to bind to the B cells, suggesting that activation was not associated with global changes in surface sialic acid content. B cell stimulation did not appear to increase the activity of the most prevalent B cell sialidase activity as measured in an in vitro assay system, suggesting that the major B cell sialidase may not be responsible for the alteration of B cell sialylation levels or the ability of activated B cells to interact more effectively with T cells. The possibility of intracellular compartmentalization of sialidase activity or that a minor B cell sialidase may play a role in the regulation of a B cells ability to interact with T cells are discussed.     

Regulation of B Cell:T Cell Interactions: Potential Involvement of an Endogenous B Cell Sialidase

In light of the ability of B cells treated with neuraminidase to interact more effectively with T cells, the increased capacity of activated, but not small resting B cells, to interact with T cells could be associated with the level of sialylation on certain B cell surface molecules which influences the effectiveness of the physical interaction between B and T cells. The purpose of this study was to determine if activation of B cells altered sialylation via an endogenous sialidase which affected both the initial interaction between T and B cells and subsequent B cell-induced T cell proliferation. The competitive neuraminidase inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (NeuAc2en), inhibited LPS-mediated enhancement of B cell conjugate formation with Ia-specific T cell clones as well as enhancement of their capacity to stimulate a mixed lymphocyte reaction. The addition of NeuAc2en during LPS stimulation did not affect the surface expression of Ia, LFA-1, ICAM-1 or mB7, suggesting that inhibition of LPS-mediated enhancement by the sialidase inhibitor was not due to changes in the level of expression of the major B cell adhesion or co-stimulatory molecules. Short term stimulation with phorbol myristate acetate (PMA) and ionomycin also enhanced the ability of resting B cells to form antigen specific T:B conjugates. However, activation of B cells with PMA and ionomycin or with LPS did not change the capacity of a sialic acid specific lectin to bind to the B cells, suggesting that activation was not associated with global changes in surface sialic acid content. B cell stimulation did not appear to increase the activity of the most prevalent B cell sialidase activity as measured in an in vitro assay system, suggesting that the major B cell sialidase may not be responsible for the alteration of B cell sialylation levels or the ability of activated B cells to interact more effectively with T cells. The possibility of intracellular compartmentalization of sialidase activity or that a minor B cell sialidase may play a role in the regulation of a B cells ability to interact with T cells are discussed.     

Alloantigen-specific T-cell anergy induced by human keratinocytes is abrogated upon loss of cell-cell contact

The activation of primary human T cells largely depends on the expression of both major histocompatibility complex (MHC) class II and B7 molecules on antigen-presenting cells (APC), whereas APC expressing HLA class II but not B7 antigens are expected to induce anergy. According to this concept, interferon-γ (IFN-γ)-activated keratinocytes (KC) expressing HLA class II but not B7 costimulatory antigens should be able to induce anergy. However, in terms of anergy versus activation contradicting data have been published on the outcome of interaction between T cells and human KC. In addition, it has been shown that human KC can express a B7-like molecule with unknown function, whereas MHC expression may be functionally impaired. To evaluate this item we transfected the human A431 KC cell line with B7-1 coding sequences and up-regulated HLA-DR by treatment with IFN-γ, yielding A431^DR,B7-1 cells. Irradiated A431^DR,B7-1 cells were found to be capable of inducing vigorous proliferative primary T-cell responses in resting allogeneic T cells, whereas A431^DR cells could induce proliferation only when interleukin-2 (IL-2) was added. These data indicate that KC can present alloantigens, and that lack of costimulatory molecules on KC is the main reason why these cells cannot induce primary T-cell responses. Surprisingly, however, no evidence could be obtained of stable anergy induction by A431^DR cells, as T cells contacted with A431^DR cells and then transferred to A431^DR,B7-1 cells clearly demonstrated alloresponsiveness. T-cell non-responsiveness was maintained only when T cells remained in contact with A431^DR cells. These data indicate that, despite expression of HLA class II in the absence of B7 costimulatory molecules, human KC cannot induce stable anergy but rather induce short-term anergy in primary resting T cells.     

ERMAP is a B7 family-related molecule that negatively regulates T cell and macrophage responses

T cell activation and tolerance are tightly regulated by costimulatory and coinhibitory molecules. B7 family members play a crucial role in regulating immune responses. In this study, we identified erythroid membrane-associated protein (ERMAP) as a novel T cell inhibitory molecule. ERMAP shares significant sequence and structural homology with existing B7 family members in its extracellular domain. The ERMAP protein is expressed on the cell surface of resting and activated antigen-presenting cells (APCs) and in some tumor tissues. The putative ERMAP receptor is expressed on activated CD4 and CD8 T cells and macrophages. Both mouse and human ERMAP-IgG2a Fc (ERMAP-Ig) fusion proteins inhibit T cell functions in vitro. Administration of ERMAP-Ig protein ameliorates autoimmune diseases, including experimental autoimmune encephalomyelitis and type 1 diabetes, in mice. Anti-ERMAP antibody enhances macrophage phagocytosis of cancer cells in vitro. Furthermore, administration of an anti-ERMAP antibody inhibits tumor growth in mice likely by blocking the inhibitory effects of ERMAP on T cells and macrophages. Our results suggest that therapeutic interaction with the ERMAP inhibitory pathway may represent a novel strategy for treating patients with autoimmune disease or cancer.     

T lymphocytes express B7 family molecules following interaction with dendritic cells and acquire bystander costimulatory properties

Dendritic cells (DC) play a pivotal role in the initiation, maintenance and regulation of the immune response. Here we obtained the first evidence that DC, in the absence of any foreign antigens, induce the expression of B7 family costimulatory molecules, such as CD80, CD86, B7-H1, PD-L2, B7-H3, and B7RP-1, on autologous T lymphocytes. Cell-to-cell contact between DC and T cells was needed in order to obtain this expression on T cells. De novo expressed B7 molecules on T cells were functional since B7+ T cells were able to costimulate the proliferation of highly purified T cells. While both autologous and allogeneic DC were able to induce similar levels of costimulatory molecule expression, the chemokine receptor repertoire on B7+ T cells after interaction with DC varied depending on the presence of allo-antigens during the interaction (CCR7-, CCR5+) or the absence of antigens (CCR7+, CCR5-). In accordance with this different pattern of chemokine receptors in the two conditions, we propose that, after the encounter with DC in lymphoid organs, this peculiar T cell population should reside in the T cell areas of the lymph nodes or migrate to peripheral sites of inflammation, providing a second signal for activating or switching off, respectively, naive or peripheral effector T cells.     

A role for adhesion molecules in contact-dependent T help for B cells.

The role of cell contact in T-dependent B cell activation was examined. Small resting B cells from C57BL/6 mice were cultured with CBA-derived, non-alloreactive cloned T helper cells in anti-T cell receptor V beta 8-coated microwells. This induced polyclonal B cell activation to enter cell cycle (as measured by thymidine incorporation at 2 days) and to secrete immunoglobulin (as measured by an enzyme-linked immunoassay detecting high-rate Ig secretion at 5 days). The inclusion of monoclonal antibodies against LFA-1. ICAM-1 and CD4 in these cultures strongly inhibited antibody responses, although proliferative responses were only inhibited to about 50%. Inhibitory monoclonal antibodies did not significantly affect lipopolysaccharide-induced responses. T cell activation to interleukin (IL) 3 secretion, nor did they inhibit the formation of multicellular clusters containing T and B cells. There was no correlation between the level of expression of adhesion molecules by T cells and their ability to induce B cell responses. Anti-LFA-1 abrogated T-dependent responses to IL2 which were inducible after 2 days in culture, but did not inhibit the induction of this IL2 responsiveness. These results suggest that continued cell contact involving adhesion/accessory molecules induces B cells to proliferate and to respond to T cell lymphokines. A signaling role for cell interaction molecules on B cells is proposed, similar to the role of these and analogous molecules on T cells.     

Regulatory role of mature B cells in a murine model of inflammatory bowel disease

The spontaneous chronic colitis in TCR alpha mutant (TCRalpha(-/-)) mice mediated by CD4(+) TCRalpha(-)beta(+) T cells is more severe in the absence of mature B cells, suggesting a suppressive role of B cells and Ig in the development of chronic colitis. To investigate the direct role of B cells in the suppression of this colitis, cell transfer studies were performed in TCRalpha(-/-) x Igmu(-/-) (alphamu(-/-)) double-knockout mice. The chronic colitis was markedly attenuated in alphamu(-/-) mice after the adoptive transfer of peripheral B cells from TCRalpha(-/-) mice into 3- to 4-week-old alphamu(-/-) mice prior to the development of colitis. Furthermore, transfer of mature B cells from TCRalpha(-/-) mice markedly decreased the number of pathogenic colonic CD4(+) TCRalpha(-)beta(+) T cells in alphamu(-/-) mice with established colitis. This B cell effect required the presence of functional co-stimulatory molecules CD40 and B7-2 (CD86) but not B7-1 (CD80). These results indicate that mature B cells play an important role in the development of chronic colitis in TCRalpha(-/-) mice by directly regulating the pathogenic T cells (CD4(+) TCRalpha(-)beta(+) T cells).     

Human Naive and Memory T-Helper Cells Display Distinct Adhesion Properties to ICAM-1, LFA-3 and B7 Molecules

In this paper the contribution of different accessory molecules to the adhesion of resting, naive and memory CD4⁺ T cells was examined utilizing a panel of CHO cell transfectants as model antigen-presenting cells (APCs). CD4⁺ T lymphocytes demonstrated strong adhesion to HLA-DR4 transfected CHO cells co-expressing B7, ICAM-I or LFA-3 molecules, suggesting that all three adhesion pathways is utilized by resting CD4⁺ cells. Monoclonal antibodies (MoAbs) against the corresponding receptors on T cells, e.g. anti-CD28, anti-LFA-1β and anti-CD2, inhibited completely T-cell adhesion to natural ligands expressed on transfected CHOcells. Pretreatment of CD4⁺ T cells with NKI-L16 MoAb, which interact with an activation epitope on LFA-loc chain, enhanced adhesion to ICAM-1 but not B7 or LFA-3 expressing CHO cells. Analysis of T helper-cell subsets revealed that memory T cells bound several fold stronger to ICAM-1 expressing transfectants compared to the CD4⁺ 45RA⁺ naive T cells, whereas adhesion to B7, LFA-3- and B7/LFA-3-expressing CHO cells was similar in both T-cell subsets. The kinetics of adhesion of naive and memory CD4⁺ T cells to ICAM-1 was rapid and similar in both subsets. The NKI-L16 MoAb multiplied several times ICAM-1-dependent adhesion in naive compared to memory cells, which enabled the naive cells to reach a similar adhesion level as memory cells. The results suggest that resting naive CD4⁺ T cells utilize preferentially the CD2/LFA-3 or CD28/B7 adhesion pathways upon adhesion to APCs, while memory CD4⁺ T cells utilize the CD2/LFA-3, CD28/B7 and LFA-l/ICAM-1 adhesion pathways. The NK.I-L16 MoAb-induced upregulation of adhesion involves an increased affinity of LFA-1 for its ligand and not a change in the number of LFA-1 molecules. This is compatible with a view that naive cells express a large number of inactive LFA-1 molecules, whereas memory cells express preferentially activated LFA-1 molecules. The inherent low number of active LFA-I molecules on naive CD4⁺ T cells may be important in keeping these cells in a resting state.     

Differential effects of B7-1 and B7-2 on the costimulation of mouse nonspecific cytotoxic T lymphocyte development in response to anti-CD3 antibody

Despite extensive study, the relative contribution of B7-1 and B7-2 molecules to the costimulation of cytotoxic T lymphocyte (CTL) activation remains controversial. We used blocking mAbs to B7-1 and B7-2 molecules to determine the role of these B7 family members in the in vitro induction of mouse nonspecific CTL in response to soluble anti-CD3 mAb. Optimal induction of anti-CD3-activated killer-T (AK-T) cells was found to require interactions with B7-2 on residual accessory cells in nylon wool-nonadherent spleen cell preparations during the first 12 h of culture in the presence of anti-CD3 mAb. Because B7-1 is not expressed at high enough levels on residual accessory cells in primary T cell cultures to be an effective ligand for CD28, we used LPS-stimulated B cells, which express substantial B7-1, in addition to B7-2, to determine the contribution of B7-1 to AK-T cell development. Compared with B7-2, the contribution of B7-1 to the costimulation of AK-T cells in this system was modest because anti-B7-1 mAb had only a minimal inhibitory effect on the generation of cytotoxicity, whereas anti-B7-2 mAb strongly inhibited AK-T cell development. Anti-CD3-induced cytotoxicity of T cells from CD4 knockout mice and CD4-depleted nylon wool-nonadherent spleen cells from wild-type mice was inhibited by anti-B7-2 mAb, implying that B7-2 is able to bind directly to CD28 on CD8+ T cells and costimulate their activation. B7-1 blockade, on the other hand, did not affect the costimulation of CD8+ T cells. Blockade of B7-2/ CD28 interactions with anti-B7-2 mAb strongly inhibited granzyme B, but not perforin or Fas ligand gene expression, suggesting an explanation for the inhibitory effect of anti-B7-2 mAb on AK-T cell development. These data indicate that B7-2 is superior to B7-1 as a costimulator of mouse AK-T cell induction.     

Immune checkpoint inhibitors in cancer therapy

In recent years immune checkpoint inhibitors have garnered attention as being one of the most promising types of immunotherapy on the horizon. There has been particular focus on the immune checkpoint molecules, cytotoxic Tlymphocyte antigen-4 (CTLA-4) and programmed cell death protein 1 (PD-1) which have been shown to have potent immunomodulatory effects through their function as negative regulators of T cell activation. CTLA-4, through engagement with its ligands B7-1 (CD80) and B7-2 (CD86), plays a pivotal role in attenuating the activation of na?ve and memory T cells. In contrast, PD-1 is primarily involved in modulating T cell activity in peripheral tissues via its interaction with PD-L1 and PD-L2. The discovery of these negative regulators of the immune response was crucial in the development of checkpoint inhibitors. This shifted the focus from developing therapies that targeted activation of the host immune system against cancer to checkpoint inhibitors, which aimed to mediate tumor cell destruction through the removal of coinhibitory signals blocking anti tumor T cell responses.