Fine specificity mapping of two allospecific T cell lines: recognition of private specificities in the H-2 IA subregion

Host immune response against DENV and ZIKV infections

Dengue is a major public health concern, affecting almost 400 million people worldwide, with about 70% of the global burden of disease in Asia. Despite revised clinical classifications of dengue infections by the World Health Organization, the wide spectrum of the manifestations of dengue illness continues to pose challenges in diagnosis and patient management for clinicians. When the Zika epidemic spread through the American continent and then later to Africa and Asia in 2015, researchers compared the characteristics of the Zika infection to Dengue, considering both these viruses were transmitted primarily through the same vector, the Aedes aegypti female mosquitoes. An important difference to note, however, was that the Zika epidemic diffused in a shorter time span compared to the persisting feature of Dengue infections, which is endemic in many Asian countries. As the pathogenesis of viral illnesses is affected by host immune responses, various immune modulators have been proposed as biomarkers to predict the risk of the disease progression to a severe form, at a much earlier stage of the illness. However, the findings for most biomarkers are highly discrepant between studies. Meanwhile, the cross-reactivity of CD8+ and CD4+ T cells response to Dengue and Zika viruses provide important clues for further development of potential treatments. This review discusses similarities between Dengue and Zika infections, comparing their disease transmissions and vectors involved, and both the innate and adaptive immune responses in these infections. Consideration of the genetic identity of both the Dengue and Zika flaviviruses as well as the cross-reactivity of relevant T cells along with the actions of CD4+ cytotoxic cells in these infections are also presented. Finally, a summary of the immune biomarkers that have been reported for dengue and Zika viral infections are discussed which may be useful indicators for future anti-viral targets or predictors for disease severity. Together, this information appraises the current understanding of both Zika and Dengue infections, providing insights for future vaccine design approaches against both viruses.

CD4+ Cytotoxic T cells - Phenotype, Function and Transcriptional Networks Controlling Their Differentiation Pathways

The two major subsets of peripheral T cells are classically divided into the CD4⁺ T helper cells and the cytotoxic CD8⁺ T cell lineage. However, the appearance of some effector CD4⁺ T cell populations displaying cytotoxic activity, in particular during viral infections, has been observed, thus breaking the functional dichotomy of CD4⁺ and CD8⁺ T lymphocytes. The strong association of the appearance of CD4⁺ cytotoxic T lymphocytes (CD4 CTLs) with viral infections suggests an important role of this subset in antiviral immunity by controlling viral replication and infection. Moreover, CD4 CTLs have been linked with anti-tumor activity and might also cause immunopathology in autoimmune diseases. This raises interest into the molecular mechanisms regulating CD4 CTL differentiation, which are poorly understood in comparison to differentiation pathways of other Th subsets. In this review, we provide a brief overview about key features of CD4 CTLs, including their role in viral infections and cancer immunity, and about the link between CD4 CTLs and immune-mediated diseases. Subsequently, we will discuss the current knowledge about transcriptional and epigenetic networks controlling CD4 CTL differentiation and highlight recent data suggesting a role for histone deacetylases in the generation of CD4 CTLs.

Cytotoxic CD4 T Cells-Friend or Foe during Viral Infection?

CD4 T cells with cytotoxic function were once thought to be an artifact due to long-term in vitro cultures but have in more recent years become accepted and reported in the literature in response to a number of viral infections. In this review, we focus on cytotoxic CD4 T cells in the context of human viral infections and in some infections that affect mice and non-human primates. We examine the effector mechanisms used by cytotoxic CD4 cells, the phenotypes that describe this population, and the transcription factors and pathways that lead to their induction following infection. We further consider the cells that are the predominant targets of this effector subset and describe the viral infections in which CD4 cytotoxic T lymphocytes have been shown to play a protective or pathologic role. Cytotoxic CD4 T cells are detected in the circulation at much higher levels than previously realized and are now recognized to have an important role in the immune response to viral infections.

The Differentiation and Protective Function of Cytolytic CD4 T Cells in Influenza Infection

CD4 T cells that recognize peptide antigen in the context of class II MHC can differentiate into various subsets that are characterized by their helper functions. However, increasing evidence indicates that CD4 cells with direct cytolytic activity (CD4 CTL) play a role in chronic as well as acute infections, such as influenza A virus (IAV) infection. In the last couple of decades, techniques to measure the frequency and activity of these cytolytic cells has demonstrated their abundance in infections, such as human immunodeficiency virus, mouse pox, murine gamma herpes virus, cytomegalovirus, Epstein-Barr virus, and influenza among others. We now appreciate a greater role for CD4 CTL as direct effectors in viral infections and antitumor immunity through their ability to acquire perforin-mediated cytolytic activity and contribution to lysis of virally infected targets or tumors. As early as the 1980s, CD4 T cell clones with cytolytic potential were identified after influenza virus infection, yet much of this early work was dependent on in vitro culture and little was known about the physiological relevance of CD4 CTL. Here, we discuss the direct role CD4 CTL play in protection against lethal IAV infection and the factors that drive the generation of perforin-mediated lytic activity in CD4 cells in vivo during IAV infection. While focusing on CD4 CTL generated during IAV infection, we pull comparisons from the literature in other antiviral and antitumor systems. Further, we highlight what is currently known about CD4 CTL secondary and memory responses, as well as vaccination strategies to induce these potent killer cells that provide an extra layer of cell-mediated immune protection against heterosubtypic IAV infection.

Immune response: self-foreignness

The classical basic concept of the immune system as a defence system per se and immunological surveillance against neoplasia have never been satisfactorily verified experimentally. The reason for this lies in the historic development of immunology in terms of observations of infectious disease and the interpretation of those observations. Thus, based on a complete lack of understanding of immune events, immunization procedures were developed by Pasteur and his contemporaries. The success of some of these immunization methods, influenced by culture and philosophical thoughts, and based on prima facie evidence allowed the next conceptual step to be taken, culminating in the immune surveillance hypothesis. Central to this hypothesis is selection and tolerance to self-antigens. However, immune reactions to self-antigens are evident and clonal selection is not viable because the number of clones required increases as the frequency of chance of a cell belonging to a particular clone decreases. Also, circadian rhythms in the immune response have not been taken into account. In addition, the problems of haemocytopoiesis have not been addressed, in that it is possible for B-lymphocytes to become terminal macrophages and T-lymphocytes to become mast cells, eosinophils and/or basophils constituting 'dead end' cells in an immune response. The initiation of the immune response begins with a tissue-specific T-lymphocyte being stimulated and undergoes replication. This gives rise to a dual functional helper/suppressor cell and a B-lymphocyte. These basic concepts explain the necessity for auto-reactive lymphocytes, that is the autologous mixed lymphocyte reaction (AMR). The AMR is a natural consequence of having tissue-specific lymphocytes to monitor plasma membrane aberrations.

Expression of specific cytolytic activity by H-2I region-restricted, influenza virus-specific T lymphocyte clones

Among murine class II major histocompatibility complex (MHC)-restricted cytotoxic T lymphocyte (CTL) clones specific for type A influenza virus, we have identified both noncytolytic clones and clones exhibiting H-2 I region-restricted cytolytic activity. After appropriate antigenic stimulation, both cytolytic and noncytolytic clones proliferated in the absence of exogenous interleukin 2. All of the clones possess the Thy-1.2+, Lyt-1+2-, L3T4+ phenotype. The class II MHC restriction of viral recognition by the CTL clones was mapped by proliferation using recombinant mouse strains and by inhibition of cytotoxic activity with monoclonal antibodies directed to class II MHC products and L3T4a. The restriction specificity of two CTL clones was unambiguously assigned to the E beta d chain by using L cell transfectant lines expressing E alpha kE beta d or E alpha kE beta k gene products. Analysis of the viral specificity of the cloned lines revealed subtype-specific and crossreactive patterns of viral antigen recognition; the pattern of viral antigen specificity exhibited by each clone in proliferation and cell-mediated cytotoxicity was identical. Each CTL clone also demonstrated antigen-dependent release of helper factor(s) that promoted in vitro primary anti-SRBC responses. Finally, the cytotoxic effector function of the class II MHC-restricted CTL clones was mediated by direct lysis of virus-infected cells, and not by secretion of a cytolytic lymphokine.

Karyotypic analysis of murine cytolytic T-cell lines

G-banding techniques have been used to analyze the karyotypes of four independently derived murine cytolytic T lymphocyte (CTL) lines established in culture 2-4 yr earlier. For two of them, several subclones were included in the analysis. The modal arm number of three of the lines was close to diploid, whereas in the fourth line, both near-diploid and near-tetraploid subclones were found. The most frequent type of rearrangement was the formation of metacentrics by centromeric fusion. A comparison of the abnormalities (markers) occurring in the different CTL lines showed the formation of similar chromosomal rearrangements in independent lines, thus suggesting nonrandom chromosomal changes in these cells. Several of the metacentrics are derived from the fusion of normal murine chromosomes that are apparently prone to form such "markers," as the occurrence of similar abnormalities has been reported in other established cell lines and tumors. On the other hand, some of the biarmed and telocentric marker chromosomes seem to characteristically occur in CTL but not in other types of cell lines.

Analysis of the repertoire of anti-idiotypic B-cell responses to self-I-Ak- or -I-Ek-reactive monoclonal antibodies in A.TL mice

Twenty anti-idiotypic antisera (anti-Ids) were produced in A.TL mice to self-I-Ak or -I-Ek-reactive monoclonal antibodies (mAbs), constructed in the A.TH anti-A.TL combination. The reactivity of these anti-Ids was examined in a panel of 31 anti-Iak A.TH mAbs, using direct idiotype binding, cross-competitive inhibition of idiotype binding, and isoelectrofocusing (IEF) assays. Among 13 anti-Ids produced against anti-I-Ak mAbs, one only recognized individual idiotypic specificities (IdIs) on its corresponding mAb, while the 12 others identified homologous IdIs and recurrent idiotypic specificities also expressed on heterologous anti-I-Ak and/or I-Ek mAbs. Two sets of major cross-reactive idiotypes (IdXs) were characterized on two groups of mAbs recognizing public Ia.1, I-Ak,f,u and r) or private (Ia.2, I-Ak) determinants clustered in two spatially distinct epitope regions of the I-Ak molecule, respectively. By contrast, most (5/7) of the anti-Ids raised against mAbs recognizing polymorphic or monomorphic (Ia.7-like) I-Ek determinants displayed specificity apparently restricted to their corresponding mAb IdIs. This finding contrasted with the previous characterization, using xenogeneic anti-idiotypic reagents, of an interstrain IdX expressed on all mAbs defining Ia.7-like determinants in the IEk epitope group I. These data indicate that A.TL mice can readily develop anti-idiotypic responses towards self Ia-reactive mAb minor idiotypes (IdIs) and that recognition of anti-Iak mAb IdXs in such mice is preferentially observed when anti-I-Ak mAbs are used as immunogens.

Evidence for a new lymphocyte-stimulating determinant (Lsd) detected by alloreactive T cell lines

Three alloreactive T cell clones are described which reveal a new lymphocyte-stimulating determinant (Lsd) controlled by a gene on chromosome 1 of the mouse but different from and possibly centromeric to Mls. Two clones were originally isolated as anti-H-2b-specific and showed and retained cross-reactivity to Lsd (clone OD3, BALB/c anti-C57BL/6; clone KB37, B10.BR anti-C57BL/10). The third clone (BD7, C57BL/6 anti-CBA/J) was probably originally directed against Lsd. So far, Lsd is only characterized by stimulation of T cell proliferation, and its physiological function is not yet known.

Alloreactive T-cell clones. Ly phenotypes predict both function and specificity for major histocompatibility complex products

We have studied the association of Ly phenotype with function and specificity for major histocompatibility complex (MHC) products by examining the properties of 21 T-cell clones derived from B10 anti-B10.D2 and B10.A anti-B10.D2 mixed lymphocyte cultures (MLC). T cells were selected after MLC solely on the basis of Ly phenotype, cloned by limiting dilution, and tested for stability of Ly phenotype, function and specificity for class I or class II MHC products. Sixteen Ly-1+2- and five Ly-1-2+ T-cell clones were tested. The clones selected for the Ly-1+2- phenotype maintained this phenotype, expressed helper but not lytic function, and recognized class II MHC products (I-Ad or I-Ed). All Ly-1-2+ clones maintained this phenotype, possessed cytolytic but not helper activity, and recognized class I MHC products (Dd and Ld). Our data therefore confirm at the clonal level the original observations of a remarkably consistent correlation between Ly markers, MHC specificity, and function. They suggest that the expression of Ly antigens on T-cell clones forms part of a genetic program for each of these specialized cells that also determines their function and MHC specificity.

Characterization of an Lyt-1+ cytolytic T-cell clone specific for a polymorphic domain of the I-Ak molecule

We have characterized a cytolytic T-cell clone, isolated from a A.TH anti-A.TL mixed lymphocyte culture, which recognized a private determinant of the I-Ak molecule. This specificity has been confirmed by inhibition of effector-target cell interaction by anti-I-Ak monoclonal antibody (mAb). Comparison of the inhibitory capacity of various mAb and the spatial arrangement of their epitopes (defined in previous studies by antibody-binding competition) indicated that the antigenic site recognized by this cytolytic T-cell clone was topologically related to one of the major polymorphic domains of the Ak molecule. This clone expressed the Thy-1.2+, Lyt-1.+, Lyt-1.2+low and I-As- cell surface phenotype. Testing of several rat mAb, screened for their ability to inhibit H-2K/D-specific cytolysis at the level of the effector cells, revealed that two anti-p94, 180 mAb but not various anti-Lyt-2 mAb inhibited the lytic function of this anti-I-Ak T-cell clone.

Cytolytic T cells activated by H-2-controlled E molecules cross-react with A molecules

Cell-mediated lymphocytotoxicity was generated in four strain combinations differing only by the cell-surface expression of the class II E molecule controlled by the H-2 complex. The four combinations were: B10.D2(R107) anti-B10.A(3R), B10.A(4R) anti-B10.A(2R), B10.GD anti-B10.D2(R101), and B10.S(7R) anti-B10.S(9R). In all four of these combinations, the stimulator expresses E molecules on the cell surface, while the responder does not. The cytolytic T lymphocytes generated in the B10.D2(R107) anti-B10.A(3R) and B10.A(4R) anti-B10.A(2R) combinations reacted not only with the stimulator but also with strains that do not express cell-surface E molecules, in particular, strains carrying the H-2f and H-2q haplotypes. The cross-reactivity with E-negative strains could be blocked by monoclonal antibodies specific for the Af or Aq molecules but not by antibodies recognizing determinants on E or class I (K) molecules. The anti-H-2f cross-reactivity could be inhibited by H-2q cold targets and, reciprocally, the anti-H-2q reactivity could be blocked by H-2f cold targets. These findings are interpreted as indicating that the cytolytic T lymphocytes stimulated by E molecules can recognize and lyse cells lacking E molecules but expressing A molecules. The observed E-A cross-reactivity supports the notion of structural and functional relatedness between the A and E molecules and suggests a common evolutionary origin of the A- and E-encoding loci.

T cells may express multiple activities: specific allohelp, cytolysis, and delayed-type hypersensitivity are expressed by a cloned T-cell line

A T-cell line that recognizes private specificities encoded in the H-2 IA subregion in the proliferative response, cytotoxic activity, delayed-type hypersensitivity, and allohelp reaction with B cells has been cloned. The individual clones were assayed for ability to express the activities of the parent line. Results show that the cloned sublines tested are able to perform all the activities of the parent line. It is suggested that T cells may express multiple activities and that the apparent monofunctionality of T cells is due to regulation of these activities.

Culture supernatants of a stimulated T-cell line have helper activity that acts synergistically with interleukin 2 in the response of B cells to antigen

Culture supernatants of an antigen-stimulated long-term alloreactive T cell line, C.C3.11.75, contain a T-cell-replacing factor (TRF) activity for the B-cell response to antigen. These same supernatants show little activity in the T-cell growth assay or the costimulator assay. TRF activity was assayed by using spleen cells that were rigorously depleted of both T cells and macrophages. In this assay preparations containing interleukin 2 and supernatants from stimulated C.C3.11.75 cells are relatively inactive if added alone but show marked synergy when added together. We conclude that the C.C3.11.75 TRF activity is not due to interleukin 1 or to interleukin 2 but to a third factor provisionally designated as (DL)TRF. This activity may be equivalent to the (late-acting) TRF described by Schimpl and Wecker. Evidence is presented suggesting that the helper activity (DL)TRF is a product of the T-cell line.