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Biomedical subjects

H W Sheppard

Publications and source records attributed to H W Sheppard.

At least 19 recordsLinked to original sources
PubMed1992

The natural history and pathogenesis of HIV infection.

Infection with human immunodeficiency virus (HIV) results in progressive deterioration of the cell-mediated immune system characterized by T-helper-cell dysfunction and loss in the face of signs of generalized immune-system activation. The final stage of HIV disease, AIDS, has a myriad of opportunistic infections and malignancies as its hallmarks. The causal relationship between HIV and this complex disease pattern is clear but the mechanisms by which it occurs are not well understood. There are a number of new developments in our understanding of the natural history of HIV infection from a laboratory standpoint. Our review of this information raises further questions as to the validity of the conventional "cytopathic" model and all its direct descendants. In response to these conflicts, we have developed and present an alternative hypothesis in which AIDS pathogenesis, in all its manifestations, is seen as the outcome of one central process, excess immune activation generated by the interaction of virus with the CD4 receptor. The implications of this hypothesis on therapy of HIV infections are discussed.

HIV Infections↗
PubMed1992

The relationship between AIDS and immunologic tolerance.

A hypothesis is presented in which the immunodeficiency and cell loss leading to acquired immune deficiency syndrome and the clonal deletion associated with immunologic tolerance occur through a common mechanism. In a previous publication we proposed that the interaction of human immunodeficiency virus (HIV) with CD4 delivers activation signals that disrupt immune system regulation. In this article, we compare the biology of HIV infection with recent discoveries concerning the "two-signal" molecular mechanism for thymic selection. We propose that two-signal activation is normally followed by clonal expansion and the programmed death of most or all daughter cells through mechanisms that are proportional to the strength and duration of the activation signals. In the thymus, self-reactive cells are trapped in the continuous presence of both antigen signals (signal 1) and costimulatory signals (signal 2), leading to clonal deletion. In mature lymphocytes, we propose that HIV infection contributes a chronic high-affinity signal 2, which shifts the equilibrium of antigen-activated T-cell populations further toward programmed death. This leads to incremental memory cell deficits and gradual clonal deletion at a rate dependent on the frequency of antigen exposure and the ability of an HIV quasispecies to induce signal 2.

Acquired Immunodeficiency Syndrome↗
PubMed1991

Production of site-selected neutralizing human monoclonal antibodies against the third variable domain of the human immunodeficiency virus type 1 envelope glycoprotein.

Cell lines secreting IgG1 human monoclonal antibodies (mAb) to the envelope glycoprotein, gp120, of human immunodeficiency virus (HIV) have been produced by transformation of peripheral blood cells from HIV-infected individuals and by fusion of transformed cells to a human-mouse heteromyeloma cell line (SHM-D33). Two human mAbs were site-selected by means of a 23-mer synthetic peptide spanning a portion of the third variable domain of gp120 from the MN strain of HIV. The two heterohybridomas produce three times more IgG than do their parent lymphoblastoid cell lines. The specificities of these mAbs have been mapped to sequences near the tip of the disulfide loop of the gp120 third variable domain, Lys-Arg-Ile-His-Ile and His-Ile-Gly-Pro-Gly-Arg, respectively. The mAbs have dissociation constants of 3.7 x 10(-6) M and 8.3 x 10(-7) M, neutralize HIVMN in vitro at nanogram levels, and bear the characteristics of antibodies associated with protective immunity in vivo.

Antibodies, Monoclonal↗
PubMed1991

Immune response to HIV p24 core protein during the early phases of human immunodeficiency virus infection.

The immune response to the p24 core antigen of human immunodeficiency virus type 1 (HIV-1) was studied in serial samples collected prospectively from 52 homosexual males in two separate cohorts from Amsterdam and San Francisco. p24 antibody levels were quantified with an antigen sandwich immunoassay using p24 recombinant antigen as capture and probe. Titers and slopes of dilution curves reflecting antibody affinity were analyzed. Only 45 of 52 men developed a measurable primary immune response to p24. In 17 (33%) patients there was a low response with maximum antibody titer below 66, shallow (low affinity) dilution curve, and 10 of the 17 became HIV antigen positive over a 2 year period. In 24 (46%) of the 52 patients titers ranged from 100-4000, steeper dilution curves were noted, and none became HIV antigen positive. Four (8%) men developed a strong immune response with high titers (greater than 12,000) and high affinity type dilution curve. Over time, after the peak immune response, antibody titer declined in some individuals related in part to the formation of immune complexes between HIV-1 p24 antigen and antibody which were dissociable. In vitro, the addition of increasing amounts of purified p24 antigen corresponded to decreasing antibody titer and a shallower dilution curve suggesting a preferential consumption of high affinity antibodies for complex formation. The magnitude of immune response to HIV-1 p24 antigen varies widely in infected homosexual men. Both the intrinsic ability to mount an immune response and immune complex formation contribute to the measurable antibody level.

Bisexuality↗
PubMed1991

Long-term human immunodeficiency virus infection in asymptomatic homosexual and bisexual men with normal CD4+ lymphocyte counts: immunologic and virologic characteristics.

From a prospective cohort study, 24 asymptomatic men were identified who had been antibody positive for human immunodeficiency virus (HIV) for at least 5 years (median = 9.1) with CD4+ lymphocyte counts greater than or equal to 400 cells/mm3. Of these "nonprogressors", 23 (96%) had evidence of HIV infection by either HIV culture or the polymerase chain reaction (PCR) for HIV DNA, although only 1 (4%) had a positive assay for HIV RNA (by PCR) and no one was positive for p24 antigen. Compared with 24 antibody-negative men and 14 men with AIDS, nonprogressors had higher CD8+ counts and lower natural killer cell activity. Nonprogressors had higher beta 2-microglobulin levels than did seronegative controls, suggesting some degree of immune system activation. Compared with men with AIDS, nonprogressors seemed to have a stronger antibody response to six different HIV-related proteins but did not differ significantly in neutralizing antibody or antibody-dependent cellular cytotoxic activity.

Adult↗
PubMed1991

Lack of detection of human immunodeficiency virus in persistently seronegative homosexual men with high or medium risks for infection.

Some studies have suggested that seronegative homosexual men who have practiced high-risk sexual behavior can carry the human immunodeficiency virus (HIV) in a silent state, detectable only by polymerase chain reaction (PCR) or selective viral culture. To assess this concept, blood specimens were studied from 59 homosexual men with recognized risk behaviors: unprotected anal receptive intercourse at least once and many lifetime sex partners. After extensive virologic studies and PCR analysis, only one virus-positive, antibody-negative individual was identified. These findings indicate that HIV virus-positive, seronegative individuals are rare.

Adult↗
PubMed1991

The initial immune response to HIV and immune system activation determine the outcome of HIV disease.

A study was conducted to assess the relative contribution of the HIV-1-specific immune response and -nonspecific immune activation to HIV disease progression. The titer of antibody to the p24 core protein and the concentration of serum neopterin were measured in 238 HIV-1-seropositive subjects in a prospective cohort study of homosexual men. Antibody titers were extremely variable among cohort participants but relatively stable over time, suggesting inherent differences in the initial immune response capacity. Neopterin concentrations were also variable at cohort entry but generally increased over time. These two markers, measured at cohort entry, had powerful and independent predictive value for the development of AIDS up to 54 months before diagnosis. Subjects with low antibody titers and high levels of neopterin, had the highest incidence of AIDS (60% over 54 months). Patients with low antibody or high neopterin alone had an intermediate risk (34% incidence) and less than 10% of those with high antibody and low neopterin developed AIDS. We propose that the initial immune response to HIV and virus-mediated immune system activation are independent and innately variable components of an individual's response to HIV infection that interact to determine the clinical outcome.

Acquired Immunodeficiency Syndrome↗
PubMed1991

An evaluation of the polymerase chain reaction in HIV-1 seronegative men.

The apparent detection of human immunodeficiency virus (HIV-1) DNA by the polymerase chain reaction (PCR) in seronegative individuals has been the subject of great concern. In this study, 324 seronegative participants in the San Francisco Men's Health Study were evaluated for evidence of infection using a PCR testing algorithm with multiple amplifications targeting different regions of the HIV-1 genome. While most PCR reactions were negative, 8.6% of the specimens showed weak reactivity with one or more primer sets. However, all were negative with at least one primer set and no definitively positive specimens were identified. This study addressed the possibility that some of these PCR reactions might represent latent infection or abortive exposure, leaving residual integrated DNA, rather than false-positive reactions. The frequency of such reactions was determined in homosexual men who have been at risk for HIV-1 infection and in exclusively heterosexual men who have little or no past exposure. The results demonstrate an identical frequency and distribution of equivocal PCR reactions in both populations. Assuming that there is minimal HIV-1 infection among seronegative heterosexual men in San Francisco, we conclude that PCR testing does not provide evidence for a reservoir of occult HIV-1 infection in seronegative homosexual men.

Adult↗
PubMed1991

A multicenter proficiency trial of gene amplification (PCR) for the detection of HIV-1.

The sensitivity and specificity of the polymerase chain reaction (PCR) for the detection of HIV-1 proviral DNA was determined in five laboratories with extensive experience in PCR testing. Five panels consisting of 105 HIV-1-seronegative specimens from regularly repeating blood donors with no risk factors for HIV infection and 99 HIV-1-seropositive and culture-positive specimens from a cohort of homosexual/bisexual men were sent under code to each laboratory. Amplification procedures and testing algorithms by which specimens were judged positive, negative, or indeterminate varied between laboratories. The average sensitivity for the five laboratories was 99.0%, with two laboratories achieving 100%. The average specificity was 94.7%, varying between 90.5 and 100%. The overall false-positive rate was 1.8%, the false-negative rate was 0.8%, and the indeterminate rate was 1.9%. Of 1,005 determinations made by the five laboratories, 32 (3.2%) were misclassifications. Most of the classification errors occurred in specimens from uninfected individuals and were distributed among the laboratories in such a way as to indicate laboratory error rather than the inherent reactivity of some samples. This emphasizes the need for standardization of PCR testing and caution in interpreting positive PCR reactions in HIV-1-seronegative persons.

DNA, Viral↗
PubMed1990

Application of a rapid microplaque assay for determination of human immunodeficiency virus neutralizing antibody titers.

To perform a human immunodeficiency virus (HIV) plaque assay in nonadherent host cells, we developed a novel technique in which HIV-infected MT-2 cells were formed into monolayers by centrifugation through molten agarose. Infection, formation of cell monolayers, and enumeration of plaques all took place in 96-well microtiter plates. When this process was preceded by 18 h of incubation of HIV with patient serum samples, neutralizing antibody titers between 1:10 and 1:5,000 could be accurately determined in patient serum samples. In addition to the determination of neutralizing antibody titers (with the use of various serum dilutions and a constant virus concentration), neutralization indices could also be determined with different virus dilutions and a single dilution of patient serum.

Cell Line↗
PubMed1990

AIDS as immune system activation. II. The panergic imnesia hypothesis.

A hypothesis is presented in which HIV infection leads to immunodeficiency through indirect subversion of critical T cell regulatory mechanisms. Acting at the T cell receptor complex (TCR), viral components (gp120) mimic the natural ligands of CD4, molecules of the major histocompatibility complex (MHC), and deliver physiologically active, inappropriate signals resulting in generalized, uncontrolled lymphocyte activation, or "panergy." Clinical manifestations of panergy include autoimmune phenomena, lymphadenopathy, hyperglobulinemia, and symptoms mediated by lymphokines. Immunologic unresponsiveness occurs early in HIV infection prior to T cell depletion because activated cells do not respond to further stimulation. Ultimately, activation disrupts T cell homeostasis by interference with the generation of memory cells ("imnesia") and leads to net T cell loss, clonal deletion, and the development of AIDS. The clinical and immunologic features of HIV disease and AIDS are reviewed from this perspective. This hypothesis is consistent with the paucity of infected T cells, the clinical findings of both AIDS-related complex (ARC) and frank AIDS, the prolonged "incubation period," and a role for antigen-specific cofactors. Based on this view of HIV pathophysiology, therapeutic modalities should avoid immune stimulation and seek to block aberrant gp120 signals at CD4 and eliminate HIV-infected cells.

Acquired Immunodeficiency Syndrome↗
PubMed1988

AIDS as immune system activation: a model for pathogenesis.

Epidemiological evidence has implicated the human immunodeficiency virus (HIV) as the etiological agent of the acquired immunodeficiency syndrome (AIDS) (Barré-Sinoussi et al., 1983; Sarngadharan et al., 1984; Kitchen et al., 1984; Levy et al., 1984). Primary infection with HIV is accompanied by an acute flu-like illness followed by a relatively long period of asymptomatic infection, the delayed appearance of lymphadenopathy and a progressive decline in immune responsiveness. Eventually, significant reduction in T4 cell number occurs along with susceptibility to a variety of opportunistic infections. It is generally accepted that this lymphotropic retrovirus causes the depletion of the helper class of T lymphocytes (T4) by direct cytopathic effects and/or the induction of autoimmune killing of T cells (Fauci, 1987). Few objections have been raised to this model despite abundant experimental evidence that viraemia and the number of infected T cells are limited and constant throughout disease progression (Levy et al., 1985; Shaw et al., 1984; Harper et al., 1986; Duesberg, 1987). We believe that the current model for HIV-induced pathology is unsatisfactory. A hypothesis is advanced in which progressive immunodeficiency is caused by indirect mechanisms without widespread T cell infection, direct virus-mediated cytopathic effects, or autoimmunity. We propose that the latency of AIDS is not due to delayed viral expression and growth but rather to the accumulation of insults to an immune system with abnormal regulatory mechanisms induced by HIV infection of macrophages.

Acquired Immunodeficiency Syndrome↗
PubMed1986

Cloning of Leishmania donovani genes encoding antigens recognized during human visceral leishmaniasis.

In this report we describe the construction and analysis of a genomic library of Leishmania donovani gene segments in the bacteriophage vector lambda gt11. This cloning vector permits the expression of parasite polypeptides as fusion products with Escherichia coli beta-galactosidase. A group of 90 clones which express L. donovani antigens have been isolated from this library using various polyvalent antisera. Many of these clones appear to encode parasite membrane antigens some of which are recognized by sera from patients with visceral leishmaniasis (kala azar). Some clones reacted with specific subsets of kala azar sera while others reacted with all kala azar sera tested. In addition, some of the clones appear to encode conserved antigens that are recognized by sera from mice immunized with L. major. Preliminary characterization of five clones indicated that each one contains a distinct genetic element and that at least four encode different fusion peptides.

Animals↗