MHC-unrestricted recognition of bacteria-infected target cells by human CD8+ cytotoxic T lymphocytes

MHC class Ib molecules bridge innate and acquired immunity

Our understanding of the classical MHC class I molecules (MHC class Ia molecules) has long focused on their extreme polymorphism. These molecules present peptides to T cells and are central to discrimination between self and non-self. By contrast, the functions of the non-polymorphic MHC class I molecules (MHC class Ib molecules) have been elusive, but emerging evidence reveals that, in addition to antigen presentation, MHC class Ib molecules are involved in immunoregulation. As we discuss here, the subset of MHC class Ib molecules that presents peptides to T cells bridges innate and acquired immunity, and this provides insights into the origins of acquired immunity.

Bacterial and host factors involved in the major histocompatibility complex class Ib-restricted presentation of Salmonella Hsp 60: novel pathway

Previously, a peptide epitope derived from the Hsp 60 molecule of Salmonella that is presented by the major histocompatibility complex (MHC) class Ib molecule Qa-1 to CD8(+) cytotoxic T cells (CTLs) was described. In the present study we investigated the Salmonella-induced processing and presentation pathway for generating this Qa-1-restricted epitope. Live bacteria and, to a lesser extent, opsonized heat-killed bacteria are able to sensitize target cells for lysis by Salmonella-specific CTL. In contrast, heat-killed bacteria cannot sensitize target cells. Presentation of the Hsp 60 epitope appears independent of bacterial internalization, because cytochalasin D does not affect presentation. Moreover, Salmonella strains defective in the InvA or InvE operon, two critical components of the type III secretion pathway, are as efficient as wild-type Salmonella enterica serovar Typhimurium in sensitizing infected targets to lysis. Collectively, these results suggest the existence of a novel antigen-processing pathway in which exogenous antigens gain access to the cytosolic MHC class I processing machinery. Considering the abundant nature of bacterial Hsp 60 and the upregulation of this protein after Salmonella infection of eukaryotic cells, this mode of antigen presentation may be particularly relevant to understanding the host defense mechanisms against gram-negative bacteria.

The involvement of class Ib molecules in the host response to infection with Salmonella and its relevance to autoimmunity

Class I molecules with limited polymorphism have been implicated in the host response to infectious agents. Following infection with Salmonella typhimurium, mice develop a CD8+ CTL response that specifically recognizes bacteria infected cells. An immunodominant component of the CTL response recognizes a peptide epitope derived from the Salmonella GroEL molecule that is presented by the non-polymorphic MHC class Ib molecule Qa-1. T cells recognizing the bacterial peptide also cross-recognize a homologous peptide from the mammalian hsp60 molecule. Since Qa-1 has a functional equivalent in humans, this observation may be relevant not only to the host response involved in clearing infection but also in understanding the link between infection with Gram-negative pathogens and autoimmune disease.

Molecular mimicry mediated by MHC class Ib molecules after infection with gram-negative pathogens

The development of many autoimmune diseases has been etiologically linked to exposure to infectious agents. For example, a subset of patients with a history of Salmonella infection develop reactive arthritis. The persistence of bacterial antigen in arthritic tissue and the isolation of Salmonella or Yersinia reactive CD8+ T cells from the joints of patients with reactive arthritis support the etiological link between Gram-negative bacterial infection and autoimmune disease. Models proposed to account for the link between infection and autoimmunity include inflammation-induced presentation of cryptic self-epitopes, antigen persistence and molecular mimicry. Several studies support molecular mimicry as a mechanism for the involvement of class II epitopes in infectious disease-induced self-reactivity. Here, we have identified an immunodominant epitope derived from the S. typhimurium GroEL molecule. This epitope is presented by the mouse H2-T23-encoded class Ib molecule Qa-1 and was recognized by CD8+ cytotoxic T lymphocytes induced after natural infection. S. typhimurium-stimulated cytotoxic T lymphocytes recognizing the GroEL epitope cross-reacted with a peptide derived from mouse heat shock protein 60 and recognized stressed macrophages. Our results indicate involvement of MHC class Ib molecules in infection-induced autoimmune recognition and indicate a mechanism for the etiological link between Gram-negative bacterial infection and autoimmunity.

Bacteria-specific cytotoxic CD8+ T cells: a missing link in the pathogenesis of the HLA-B27-associated spondylarthropathies

The term seronegative spondylarthropathies is used for an entity of rheumatic syndromes of peripheral joints and the spine (ankylosing spondylitis, reactive arthritis, Reiter's syndrome, arthritis in psoriasis and in inflammatory bowel disease) which are strongly associated with the MHC class I molecule HLA-B27. However, the mechanisms whereby HLA-B27 confers disease susceptibility have so far remained unknown. There is strong evidence that gut inflammation and infection with gram-negative bacteria play a role in the induction of B27-associated disease. HLA-B27, like other MHC class I molecules, physiologically binds antigenic peptides in its binding groove and presents them to CD8+ T lymphocytes. Consequently, if the disease association with HLA-B27 arises from its role as a T-cell restriction element, synovial fluid CD8+ rather than CD4+ T cells should play a prominent pathogenetic role and should be detectable within the affected joints. In this paper, recent studies on bacteria-specific cytotoxic T cells and on peptide binding to HLA-B27 are reviewed. Particular emphasis is laid on the role of HLA-B27 restricted synovial CD8+ T cells with specificity for bacterial antigens or autoantigens. These cytotoxic T cells could provide a missing link in the pathogenesis of the spondylarthropathies and could now serve as tools to identify the critical antigenic epitopes of bacterial and self peptides which are involved in disease induction.

HLA-B27-restricted CD8 T cells derived from synovial fluids of patients with reactive arthritis and ankylosing spondylitis

Ankylosing spondylitis and seronegative spondylarthropathies such as Reiter's syndrome and reactive arthritis are strongly associated with HLA-B27. However, the mechanisms by which HLA-B27 is involved in disease susceptibility and pathogenesis are unknown. If the disease association is a consequence of HLA-B27's physiological function in antigen presentation, the disease should be mediated by cytotoxic T lymphocytes (CTLs) that recognise bacterial or self peptides presented by HLA-B27. Proof of this arthritogenic peptide model requires isolation of B27-restricted CD8 T cells from arthritic joints of patients with spondylarthropathies. An important question is whether "arthritogenic" bacteria such as yersinia or salmonella can generate HLA-B27-restricted bacteria-specific CTLs. We describe such HLA-B27-restricted CTLs. We tested a panel of 354 alpha beta-TCR CD8 T lymphocyte clones (TLCs) that had been derived from the synovial fluid of 4 patients with reactive arthritis and 2 patients with ankylosing spondylitis. In 1 patient with yersinia-induced arthritis, 2 TLCs were identified that killed specifically yersinia-infected B27 target cells. In another patient with salmonella-induced arthritis, 1 B27-restricted CD8 TLC that recognised both salmonella and yersinia was identified. In 5 of the 6 patients autoreactive CTLs were found, 5 of which showed B27-restricted killing of uninfected cell lines. B27-restricted CTLs with specificity for arthritogenic bacteria or autoantigens provide a missing link in the pathogenesis of the HLA-B27-associated spondylarthropathies.

Lysis of CD4+ lymphocytes by non-HLA-restricted cytotoxic T lymphocytes from HIV-infected individuals

Individuals infected with HIV have elevated numbers of total and activated CD8+ lymphocytes in peripheral blood. CD8+ lymphocytes from HIV-infected individuals have been shown to mediate non-human histocompatibility-linked antigen (HLA)-restricted suppression of viral replication, HLA-restricted killing of cells expressing HIV antigens, and killing of uninfected lymphocytes. We studied CD8+ T lymphocytes that lysed autologous CD4+ lymphocytes. heterologous CD4+ lymphocytes from HIV-infected individuals and uninfected CD4+ lymphocytes. Killing in all cases required T cell receptor (TCR)-mediated recognition or triggering. However, these CD8+ cytotoxic T lymphocytes (CTL) killed HLA class I mismatched CD4+ lymphocytes and CD4+ lymphocytes treated with a MoAb against HLA-A, B and C antigens (PA2.6) which blocks HLA class I-restricted killing. HLA class II-negative CD4+ T lymphoma cells (CEM.NKR) were also killed by anti-CD3 inhibited CTL. Stimulation of peripheral blood lymphocytes (PBL) from HIV-infected individuals, but not uninfected controls, with concanavalin A (Con A) and IL-2, induced non-HLA-restricted TCR alpha beta+, CD8+ CTL which lysed CD4+ lymphocytes. Activation of CD4+ lymphocytes increased their susceptibility to CD8+ CTL-mediated lysis. In HIV infection, a population of non-HLA-restricted CTL which lyse activated CD4+ lymphocytes is expanded. The expansion of CTL with unusual characteristics is interesting, because the stimulus for this expansion is unknown. CTL which recognize activated CD4+ cells could play a role in immune regulation and the pathogenesis of AIDS.

Do nonclassical, class Ib MHC molecules present bacterial antigens to T cells?

Immune responses to bacterial antigens that appear unrestricted by the MHC may involve oligomorphic MHC class Ib molecules. One example is H-2M3, which binds N-formylated peptides and presents a Listeria peptide to cytotoxic T cells from infected mice. Lack of polymorphism makes these molecules a promising target for peptide vaccines.

T cells in reactive arthritis

T cells appear to play a major role in the development, maintenance and also resolution of reactive arthritis (ReA). Recent advances in understanding the processes involved in T cell activation now allow us to examine the peripheral blood and synovial fluid T cell responses to given "arthritogenic" microorganisms in terms of antigen specificity, epitope identification, cytokine secretion patterns, HLA restriction and the role of different T cell subsets in ReA. Peripheral blood bulk proliferation and limiting dilution studies provide evidence that the peripheral T cell response against arthritis-associated gram-negative bacteria is decreased in patients developing immunological sequelae such as ReA after gastrointestinal infection. Using clonal analysis of synovial fluid CD4+ T cells it has been shown that a polyclonal rather than an oligoclonal response to a variety of bacterial antigens is induced at the site of synovitis and that these CD4+ T cells produce a Th1-type of cytokine. 65 kD heat shock protein may represent one of the possible linkages of anti-infectious and autoimmune reactions. Furthermore, a spectrum of killer cells is present in the synovial fluid of patients with ReA. This spectrum of cytotoxic T cells includes antigen-specific, class I-restricted alpha beta-TCR+CD8+ lymphocytes, antigen-specific, apparently non-MHC-restricted alpha beta-TCR+CD8+ lymphocytes and gamma delta-TCR+ cells with braod cytolytic activity directed against bacteria-infected target cells. HLA-B27-restricted Yersinia- or Salmonella-specific synovial fluid CD8+ T cells may provide the missing link between genetic disposition (HLA-B27) and extra-articular infection with arthritogenic bacteria in these patients.