Peter C. Doherty

MHC-restricted cytotoxic T cells: studies on the biological role of polymorphic major transplantation antigens determining T-cell restriction-specificity, function, and responsiveness.

Publisher Summary This chapter focuses on the important discovery that virus-specific cytotoxic T cells are dually specific for virus and for a self cell surface antigen encoded by the major histocompatibility complex (MHC). The initial work was carried out on the lymphocytic choriomeningitis virus system but it soon became evident that the same phenomenon applied to many other viruses. In addition, the same principle has been found to hold for other antigenic systems, such as trinitrophenyl coupled to cells, minor histocompatibility antigens, and the H-Y model. Graft rejection and the need for genetically homogeneous inbred mouse strains for cancer research led to the development of transplantation immunology and immunogenetics. The result is that the gene complex coding for major transplantation antigens is one of the better understood mammalian genetic regions. Cytotoxic T-cell specificity is comparable to serological specificity. Because quantification of specificity or cross-reactivity is difficult, and because of the technical limitations of these cytotoxic T-cell assays, results are interpreted with great reservation. MHC restriction reflects the fact that the effector function of T cells is determined by the kind of Self-H recognized together with the foreign antigen on cell surfaces: K and D are receptors for lytic signals, I determinants are receptors for cell differentiation signals that are delivered antigen-specifically by T cells.

Virus-Specific CD8+ T Cells in Primary and Secondary Influenza Pneumonia

Virus-specific CD8+ effector T cells (eCTL) are enriched in the lungs of mice with primary influenza pneumonia, though later detection of memory T cells (mCTL) in the mediastinal lymph nodes (MLN) or spleen by peptide-based staining protocols is at the limits of flow cytometric analysis. Respiratory challenge with an H3N2 virus months after H1N1 priming induces a massive recall response, which reduces virus titers 2-3 days earlier than in nave controls. Influenza-specific mCTL produce interferon-gamma within 6 hr, but still take 4-5 days to localize to the infected respiratory tract. The delay reflects that the recall response develops first in the MLN, which contains relatively few mCTL. The response to a subdominant epitope is less obvious after secondary challenge.

Defective Lymphoid Development in Mice Lacking Jak3

The Janus tyrosine kinases (Jaks) play a central role in signaling through cytokine receptors. Although Jak1, Jak2, and Tyk2 are widely expressed, Jak3 is predominantly expressed in hematopoietic cells and is known to associate only with the common γ (γc) chain of the interleukin (IL)-2, IL-4, IL-7, IL-9, and IL-15 receptors. Homozygous mutant mice in which the Jak3 gene had been disrupted were generated by gene targeting. Jak3-deficient mice had profound reductions in thymocytes and severe B cell and T cell lymphopenia similar to severe combined immunodeficiency disease (SCID), and the residual T cells and B cells were functionally deficient. Thus, Jak3 plays a critical role in γc signaling and lymphoid development.

The Intracellular Sensor NLRP3 Mediates Key Innate and Healing Responses to Influenza A Virus via the Regulation of Caspase-1

Virus-induced interlukin-1beta (IL-1beta) and IL-18 production in macrophages are mediated via caspase-1 pathway. Multiple microbial components, including viral RNA, are thought to trigger assembly of the cryopyrin inflammasome resulting in caspase-1 activation. Here, we demonstrated that Nlrp3(-/-) and Casp1(-/-) mice were more susceptible than wild-type mice after infection with a pathogenic influenza A virus. This enhanced morbidity correlated with decreased neutrophil and monocyte recruitment and reduced cytokine and chemokine production. Despite the effect on innate immunity, cryopyrin-deficiency was not associated with any obvious defect in virus control or on the later emergence of the adaptive response. Early epithelial necrosis was, however, more severe in the infected mutants, with extensive collagen deposition leading to later respiratory compromise. These findings reveal a function of the cryopyrin inflammasome in healing responses. Thus, cryopyrin and caspase-1 are central to both innate immunity and to moderating lung pathology in influenza pneumonia.

Effector CD4+ and CD8+ T-cell mechanisms in the control of respiratory virus infections.

The rules for T‐cell‐mediated control of viruses that infect via the respiratory mucosae show both common themes and differences depending on the nature of the pathogen. Virus‐specific CD8+ cytotoxic T lymphocytes (CTLs) are the key effectors of virus clearance in mice infected with both negative strand RNA viruses (influenza and Sendai) and a DNA virus, the murine γ‐herpesvirus68 (MHV‐68). Recently completed experiments establish that these activated CD8+ T cells indeed operate primarily via contact‐dependent lysis, Perform‐mediated cytotoxicity seems to be the preferred mode, though a Fas‐based mechanism can apparently serve as an alternative mechanism. Immune CD4+ T cells functioning in the absence of the CD8+ subset cannot eliminate MHV‐68 from lung epithelial cells, are somewhat less efficient than the CD8+ CTLs at clearing the RNA viruses, and are generally ineffectual in mice that lack B lymphocytes. Though cytokine secretion by CD4+ and CD8+ T cells in the virus‐infected king may promote both T‐cell extravasation and macrophage activation, such processes are not alone sufficient to deal consistently with any d these infections. However, CD4+ T help is mandatory for an effective B‐cell response, and can operate lo promote the clonal expansion of virus‐specific CD8+ T cells in the lymph nodes and spleen. Furthermore, a concurrent CD4+ T‐cell response seems to be essential for maintaining continued CD8+ T‐cell surveillance and effector capacity through the persistent, latent phase of MHV‐68 infection in B cells. Thus, the evidence to date supports a very traditional view: CD8+ T cells function mainly as killers and the CD4+ T cells as helpers in these respiratory virus infections.

Altered peptidase and viral-specific T cell response in LMP2 mutant mice.

MHC class I molecules present peptides generated by processing of endogenously synthesized proteins to CD8+ T lymphocytes. Recently, large proteolytic complexes, termed proteasomes, were implicated in antigen processing. Two proteasomal subunits, LMP2 and LMP7, are encoded within the MHC class II region, but their precise role in antigen processing is unknown. We have generated mice that harbor a disruption in their LMP2 gene. Proteasomes purified from spleen and liver of these mutant mice exhibit altered peptidase activities, and antigen-presenting cells showed reduced capacity to stimulate a T cell hybridoma specific for H-2Db plus a nucleoprotein epitope of an influenza A virus. The mutant mice have reduced (60%-70% of wild type) levels of CD8+ T lymphocytes and generate 5- to 6-fold fewer influenza nucleoprotein-specific cytotoxic T lymphocyte precursors. These findings indicate that LMP2 influences antigen processing.

Cell-mediated protection in influenza infection.

Cell-mediated immune responses should be considered in vaccination protocols.

A biological role for the major histocompatibility antigens.

A central function of the major histo-compatibility (H) antigens may be to signal changes in self to the immune system. Virus-induced modification of strong transplantation antigens apparently results in recognition by thymus-derived lymphocytes (T cells), with subsequent clonal expansion and immune elimination of cells bearing non-self determinants. The extreme genetic polymorphism found in the major H antigen systems of higher vertebrates may reflect evolutionary pressure exerted by this immunological surveillance mechanism.

TNF/iNOS-producing dendritic cells are the necessary evil of lethal influenza virus infection

Respiratory infection with highly pathogenic influenza A viruses is characterized by the exuberant production of cytokines and chemokines and the enhanced recruitment of innate inflammatory cells. Here, we show that challenging mice with virulent influenza A viruses, including currently circulating H5N1 strains, causes the increased selective accumulation of a particular dendritic cell subset, the tipDCs, in the pneumonic airways. These tipDCs are required for the further proliferation of influenza-specific CD8+ T cells in the infected lung, because blocking their recruitment in CCR2−/− mice decreases the numbers of CD8+ effectors and ultimately compromises virus clearance. However, diminution rather than total elimination of tipDC trafficking by treatment with the peroxisome proliferator-activated receptor-γ agonist pioglitazone moderates the potentially lethal consequences of excessive tipDC recruitment without abrogating CD8+ T cell expansion or compromising virus control. Targeting the tipDCs in this way thus offers possibilities for therapeutic intervention in the face of a catastrophic pandemic.

A question of self-preservation: immunopathology in influenza virus infection

Influenza A viruses that circulate normally in the human population cause a debilitating, though generally transient, illness that is sometimes fatal, particularly in the elderly. Severe complications arising from pandemic influenza or the highly pathogenic avian H5N1 viruses are often associated with rapid, massive inflammatory cell infiltration, acute respiratory distress, reactive hemophagocytosis and multiple organ involvement. Histological and pathological indicators strongly suggest a key role for an excessive host response in mediating at least some of this pathology. Here, we review the current literature on how various effector arms of the immune system can act deleteriously to initiate or exacerbate pathological damage in this viral pneumonia. Generally, the same immunological factors mediating tissue damage during the anti‐influenza immune response are also critical for efficient elimination of virus, thereby posing a significant challenge in the design of harmless yet effective therapeutic strategies for tackling influenza virus.