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Biomedical subjects

A J McMichael

Publications and source records attributed to A J McMichael.

At least 19 recordsLinked to original sources
PubMed2001

Early effects of climate change: do they include changes in vector-borne disease?

The world's climate appears now to be changing at an unprecedented rate. Shifts in the distribution and behaviour of insect and bird species indicate that biological systems are already responding to this change. It is well established that climate is an important determinant of the spatial and temporal distribution of vectors and pathogens. In theory, a change in climate would be expected to cause changes in the geographical range, seasonality (intra-annual variability), and in the incidence rate (with or without changes in geographical or seasonal patterns). The detection and then attribution of such changes to climate change is an emerging task for scientists. We discuss the evidence required to attribute changes in disease and vectors to the early effects of anthropogenic climate change. The literature to date indicates that there is a lack of strong evidence of the impact of climate change on vector-borne diseases (i.e. malaria, dengue, leishmaniasis, tick-borne diseases). New approaches to monitoring, such as frequent and long-term sampling along transects to monitor the full latitudinal and altitudinal range of specific vector species, are necessary in order to provide convincing direct evidence of climate change effects. There is a need to reassess the appropriate levels of evidence, including dealing with the uncertainties attached to detecting the health impacts of global change.

Animals↗
PubMed2001

Dysfunctional Epstein-Barr virus (EBV)-specific CD8(+) T lymphocytes and increased EBV load in HIV-1 infected individuals progressing to AIDS-related non-Hodgkin lymphoma.

Acquired immunodeficiency syndrome-related non-Hodgkin lymphomas (AIDS-NHL) are thought to arise because of loss of Epstein-Barr Virus (EBV)-specific cellular immunity. Here, an investigation was done to determine whether cellular immunity to EBV is lost because of physical loss or dysfunction of EBV-specific cytotoxic T cells. Data on EBV-specific cellular immunity were correlated with EBV load. For comparison, individuals who progressed to AIDS with opportunistic infections (AIDS-OI) and long-term asymptomatics (LTAs) were studied. The number of virus-specific T cells was detected using tetrameric HLA-EBV-peptide complexes; function of these EBV-specific T cells was determined using the interferon-gamma (IFN-gamma) Elispot assay. It was observed that EBV-specific CD8(+) T cells were present in normal numbers in human immunodeficiency virus (HIV)-infected individuals. However, their functional capacity was decreased compared with HIV(-) individuals. In AIDS-NHL patients, EBV-specific T cells were not physically lost in the course of HIV-1 infection but showed progressive loss of their capability to produce IFN-gamma in response to EBV peptides. This loss of function correlated with lower CD4(+) T-cell numbers and was accompanied by increasing EBV load. In HIV-1-infected LTA individuals, in whom CD4(+) T-cell numbers were maintained, and progressors to AIDS-OI, IFN-gamma-producing EBV-specific T cells were stable and EBV load remained stable or decreased in the course of HIV infection, suggestive of immune control. Our data indicate that functional loss of EBV-specific CD8(+) T cells with a concomitant increase in EBV load may play a role in the pathogenesis of AIDS-NHL.

AIDS-Related Opportunistic Infections↗
PubMed2001

Cytotoxic T-cell abundance and virus load in human immunodeficiency virus type 1 and human T-cell leukaemia virus type 1.

The correlation between virus load and specific cytotoxic T-lymphocyte (CTL) frequency during the chronic phase in human immunodeficiency virus type 1 (HIV-1) infection has been found to be negative in cross-sectional studies. We report here that, in infection with the related retrovirus human T-cell leukaemia virus type 1 (HTLV-1), the correlation is positive in asymptomatic carriers and zero in patients with the associated inflammatory disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). We demonstrate that the direction of the correlation may depend on the efficacy of the CTL response using mathematical models. We conclude that the CTL response is effective in asymptomatic carriers of HTLV-1, but ineffective in patients with HAM/TSP. Virus-mediated impairment of specific CTL production in HIV-1 infection can account for the negative correlation observed.

Carrier State↗
PubMed2001

Cellular immune responses to HIV.

The cellular immune response to the human immunodeficiency virus, mediated by T lymphocytes, seems strong but fails to control the infection completely. In most virus infections, T cells either eliminate the virus or suppress it indefinitely as a harmless, persisting infection. But the human immunodeficiency virus undermines this control by infecting key immune cells, thereby impairing the response of both the infected CD4+ T cells and the uninfected CD8+ T cells. The failure of the latter to function efficiently facilitates the escape of virus from immune control and the collapse of the whole immune system.

AIDS Vaccines↗
PubMed2001

Skewed maturation of memory HIV-specific CD8 T lymphocytes.

Understanding the lineage differentiation of memory T cells is a central question in immunology. We investigated this issue by analysing the expression of the chemokine receptor CCR7, which defines distinct subsets of naive and memory T lymphocytes with different homing and effector capacities and antiviral immune responses to HIV and cytomegalovirus. Ex vivo analysis of the expression of CD45RA and CCR7 antigens, together with in vitro analysis of the cell-division capacity of different memory CD8+ T-cell populations, identified four subsets of HIV- and CMV-specific CD8+ T lymphocytes, and indicated the following lineage differentiation pattern: CD45RA+ CCR7+ --> CD45RA- CCR7+ --> CD45RA- CCR7- --> CD45RA+ CCR7-. Here we demonstrate through analysis of cell division (predominantly restricted to the CCR7+ CD8+ T-cell subsets) that the differentiation of antigen-specific CD8+ T cells is a two-step process characterized initially by a phase of proliferation largely restricted to the CCR7+ CD8+ cell subsets, followed by a phase of functional maturation encompassing the CCR7- CD8+ cell subsets. The distribution of these populations in HIV- and CMV-specific CD8+ T cells showed that the HIV-specific cell pool was predominantly (70%) composed of pre-terminally differentiated CD45RA- CCR7- cells, whereas the CMV-specific cell pool consisted mainly (50%) of the terminally differentiated CD45RA+ CCR7- cells. These results demonstrate a skewed maturation of HIV-specific memory CD8+ T cells during HIV infection.

Adult↗
PubMed2001

Clustered mutations in HIV-1 gag are consistently required for escape from HLA-B27-restricted cytotoxic T lymphocyte responses.

The immune response to HIV-1 in patients who carry human histocompatibility leukocyte antigen (HLA)-B27 is characterized by an immunodominant response to an epitope in p24 gag (amino acids 263-272, KRWIILGLNK). Substitution of lysine (K) or glycine (G) for arginine (R) at HIV-1 gag residue 264 (R264K and R264G) results in epitopes that bind to HLA-B27 poorly. We have detected a R264K mutation in four patients carrying HLA-B27. In three of these patients the mutation occurred late, coinciding with disease progression. In another it occurred within 1 yr of infection and was associated with a virus of syncytium-inducing phenotype. In each case, R264K was tightly associated with a leucine to methionine change at residue 268. After the loss of the cytotoxic T lymphocyte (CTL) response to this epitope and in the presence of high viral load, reversion to wild-type sequence was observed. In a fifth patient, a R264G mutation was detected when HIV-1 disease progressed. Its occurrence was associated with a glutamic acid to aspartic acid mutation at residue 260. Phylogenetic analyses indicated that these substitutions emerged under natural selection rather than by genetic drift or linkage. Outgrowth of CTL escape viruses required high viral loads and additional, possibly compensatory, mutations in the gag protein.

Arginine↗
PubMed2001

Reconstitution of antigen presentation in HLA class I-negative cancer cells with peptide-beta2m fusion molecules.

Engineered MHC-peptide targets capable of inducing recognition by CTL may prove useful in designing vaccines for infectious disease and cancer. We tested whether peptides directly linked to beta2-microglobulin (beta2m) could complex with human HLA class I heavy chain, and could be recognized by human CTL, both as soluble reagents and as cell surface constituents. An HLA-A2-restricted peptide epitope was physically linked to the N terminus of human beta2m. This fusion protein refolded efficiently in vitro with HLA-A2 heavy chain, and when multimerized, the resultant complexes ("fusamers") bound specifically to appropriate CTL clones. These fused peptide/MHC complexes were as efficient as standard tetrameric peptide/MHC complexes in recognizing antigen-specific CTL. When the fusion protein was delivered to target cells using a retroviral vector, these cells were recognized and killed by appropriate CTL clones. Efficient sensitization to CTL lysis was achieved in TAP-negative and beta2m-negative cell lines, as well as in unmutated B cell lines, proving that such constructs may be effective in inducing CTL even when the MHC class I pathway has been disrupted. Specific peptides covalently linked to beta2m and delivered via retroviral vectors may be useful reagents for in vivo priming of CTL against epitopes of clinical relevance.

Antigen Presentation↗
PubMed2001

Epitope specificity of clonally expanded populations of CD8+ T cells found within the joints of patients with inflammatory arthritis.

OBJECTIVE: To investigate the hypothesis that clonality of synovial T cells from patients with rheumatoid arthritis is at least partly due to the presence of virus-specific T cells expressing a restricted repertoire of T cell receptors (TCRs). METHODS: Using fluorescently labeled HLA class I-peptide tetramers, populations of virus-specific CD8+ T cells were identified in samples of peripheral blood and synovial fluid taken from 4 patients with inflammatory arthritis. The TCR repertoire of the virus-specific T cells in the synovial fluid was analyzed using a panel of TCR beta variable region-specific monoclonal antibodies. Where T cells expressing a particular Vbeta chain dominated the response to a viral epitope, the sequences of these Vbeta chains were derived from sorted populations of antigen-specific T cells by reverse transcription-polymerase chain reaction. RESULTS: CD8+ T cells specific for Epstein-Barr virus, cytomegalovirus, and influenza virus were enriched in synovial fluid compared with peripheral blood. Clonal or oligoclonal populations of CD8+ T cells were found to dominate the responses to these viral epitopes in synovial fluid. CONCLUSION: The results support the hypothesis that restricted T cell receptor usage by large populations of virus-specific T cells provides one explanation for the presence of clonally expanded CD8+ T cells within the joints of patients with inflammatory arthritis. Thus, T cell clonality at a site of inflammation may reflect enrichment for memory T cells specific for foreign antigens, rather than proliferation of autoreactive T cells specific for self antigens.

Adult↗
PubMed2001

A novel approach to antigen-specific deletion of CTL with minimal cellular activation using alpha3 domain mutants of MHC class I/peptide complex.

In this study, we have compared the effector functions and fate of a number of human CTL clones in vitro or ex vivo following contact with variant peptides presented either on the cell surface or in a soluble multimeric format. In the presence of CD8 coreceptor binding, there is a good correlation between TCR signaling, killing of the targets, and FasL-mediated CTL apoptosis. Blocking CD8 binding using alpha3 domain mutants of MHC class I results in much reduced signaling and reduced killing of the targets. Surprisingly, however, FasL expression is induced to a similar degree on these CTLs, and apoptosis of CTL is unaffected. The ability to divorce these events may allow the deletion of antigen-specific and pathological CTL populations without the deleterious effects induced by full CTL activation.

Apoptosis↗
PubMed2001

Virus infections: escape, resistance, and counterattack.

Many viruses establish life-long infections in their natural host with few if any clinical manifestations. The relationship between virus and host is a dynamic process in which the virus has evolved the means to coexist by reducing its visibility, while the host immune system attempts to suppress and eliminate infection without damage to itself. This short review describes a variety of strategies that are employed by viruses to evade host immune responses. These include virus-associated escape from T cell recognition, and resistance to apoptosis and counterattack, with special reference to two papers published in this issue of Immunity (Mueller et al., 2001; Raftery et al., 2001).

Animals↗
PubMed2001

Epidemiologic associations between occupational and environmental exposures and autoimmune disease: report of a meeting to explore current evidence and identify research needs.

To advance understanding of autoimmunity associated with exposure to environmental factors, an "Exploratory Meeting Epidemiology on Occupational and Environmental Factors Associated with Autoimmunity" was organized in Bilthoven, the Netherlands, from May 10-12, 2000. Even if no firm conclusions can be drawn on a role of certain chemicals in the environment and in the work place in causing or exacerbating autoimmune responses and illnesses, many indications of this to occur exist. The aim of the meeting was to determine the optimal methodology for assessment of autoimmunity associated with occupational or environmental exposures in the human population, and to set up interdisciplinary and collaborative epidemiological studies to investigate the association of exposure to silica, hexachlorobenzene, ultraviolet radiation, and other agents with autoimmunity and autoimmune diseases in the human population. These agents were selected as carrying particular suspicion at present. It was concluded that there is a need for experimental studies in laboratory animals and for clinical investigations to improve scientific knowledge about the causes and mechanisms of environmentally-induced autoimmune disorders and their treatment; in addition there is a need for an interdisciplinary approach to epidemiological studies of the environmental and other causes of these disorders in human populations. Specific designs for epidemiological studies in this context, as well as laboratory assays for health outcomes, were reviewed. Several recommendations for the epidemiological approach to evaluating effects of environmental or occupational agents on autoimmunity were made. The prime recommendations are the following: 1) systematic descriptive epidemiological data on autoimmunity and autoimmune disorders are required; 2) the establishment of disease-reporting registries should be encouraged; 3) the development of internationally accepted standard diagnostic criteria for all autoimmune diseases should be encouraged; 4) the social impact of these disorders should be evaluated and estimations of direct and indirect economic costs should also be made; 5) the methods of exposure assessment used in epidemiological studies should be standardized; 6) laboratory methods for measurement of biological responses should be standardized; and 7) the inclusion of indicators of autoimmunity and autoimmune diseases and of relevant environmental exposures in ongoing epidemiological studies should be encouraged. The importance of studying environmental causes of autoimmune diseases and autoimmunity lies in the identification and prevention of risks to the public health, and in improving our knowledge of basic mechanisms of health and disease.

Animals↗
PubMed2001

Impact of climatic and other environmental changes on food production and population health in the coming decades.

World population will reach an estimated nine billion by 2050. Given this factor and continued economic development in today's low-income countries, the total global demand for food will increase approximately threefold over the coming half-century. Meanwhile, against this background, newly-occurring global environmental changes such as climate change are anticipated to affect food production. Other incipient large-scale environmental changes likely to affect food production include stratospheric O3 depletion, the accelerating loss of biodiversity (with knock-on effects on crop and livestock pest species) and the perturbation of several of the great elemental cycles of N and S. The ways in which these various environmental influences affect the production of food (crops and livestock on land, and wild and cultivated fisheries) are complex and interactive. Uncertainties therefore persist about how global climate change is likely to affect world and regional food production. On balance, recent modelling-based estimates indicate that, in the medium to longer term, if not over the next several decades, climate change is likely to affect crop yields adversely, especially in food-insecure regions. The prospect of increased climatic variability further increases the risks to future food production. Given these possible though uncertain adverse impacts of climatic and other environmental changes on world food production, there is a need to apply the Precautionary Principle. There are finite, and increasingly evident, limits to agro-ecosystems and to wild fisheries. Our capacity to maintain food supplies for an increasingly large and increasingly expectant world population will depend on maximising the efficiency and sustainability of production methods, incorporating socially-beneficial genetic biotechnologies, and taking pre-emptive action to minimise detrimental ecologically-damaging global environmental changes.

Agriculture↗
PubMed2001

Late seroconversion in HIV-resistant Nairobi prostitutes despite pre-existing HIV-specific CD8+ responses.

Resistance to HIV infection in a small group of Kenyan sex workers is associated with CD8+-lymphocyte responses to HIV cytotoxic T-lymphocyte (CTL) epitopes. Eleven prostitutes meeting criteria for HIV resistance seroconverted between 1996 and 1999. The occurrence and specificity of preexisting HIV-1 epitope-specific responses were examined using the IFN-gamma enzyme-linked immunospot assay, and any epitopes recognized were cloned and sequenced from the infecting viral isolate. Immunologic and behavioral variables were compared between late seroconverters and persistently uninfected sex worker controls. HIV-1 CTL epitope responses were present in four of six cases, 5-18 months before seroconversion, and their presence was confirmed by bulk CTL culture. A possible viral escape mutation was found in one of six epitopes. The key epidemiologic correlate of late seroconversion was a reduction in sex work over the preceding year. In persistently uninfected controls, a break from sex work was associated with a loss of HIV-specific CD8+ responses. Late seroconversion may occur in HIV-1-resistant sex workers despite preceding HIV-specific CD8+ responses. Seroconversion generally occurs in the absence of detectable CTL escape mutations and may relate to the waning of HIV-specific CD8+ responses due to reduced antigenic exposure.

Adult↗