Biomedical subjects

A N Akbar

Publications and source records attributed to A N Akbar.

At least 73 records · Page 4Linked to original sources

PubMed1994

The progressive differentiation of primed T cells is associated with an increasing susceptibility to apoptosis.

Recent studies have suggested that T cell memory for recall antigens resides in clones of primed T cells with a short inter-mitotic half-life. In humans such cells express an isoform of the leukocyte common antigen termed CD45RO. Nevertheless, little is known of the fate of these primed T cells after initial activation, since no markers are available to distinguish recently primed cells from long-established clones. This report is focused on a spectrum of primed CD4+ T cells characterized by an inverse relationship between the expression of two CD45 epitopes: CD45RB and CD45RO. We show that primed CD4+ T cells progress through many cycles of division from a CD45RBbrightOdull to a CD45RBdullObright state, resulting in a highly skewed distribution of the T cell receptor variable region usage within this particular population. The progressive differentiation defined by the shift from CD45RBbright to CD45RBdull is paralleled by the gradual loss of bcl-2 and gain of Fas expression, two features associated with an increased propensity for apoptosis. At the same time, the highly differentiated CD45RBdull cells selectively lose the capacity to synthesize interleukin (IL)-2, a cytokine which is particularly effective in preventing T cell apoptosis, although they produce high levels of IL-4. The inability to produce adequate levels of IL-2 leads to the apoptosis of primed CD45RBdull cells, when they are stimulated in the absence of exogenous IL-2. These observations show the crucial dependence of highly differentiated T cells on the availability of exogenous IL-2, and suggest both a major constraint for the persistence of T cell memory maintained by continually cycling primed cells, and an important mechanism contributing to the maintenance of T cell homeostasis in vivo.

Apoptosis↗

PubMed1994

Lymphocyte activation in HIV-1 infection. II. Functional defects of CD28- T cells.

OBJECTIVES AND DESIGN: The expression of the accessory molecule CD28 was compared in various populations of T and natural killer (NK) cells from HIV-1-negative and HIV-1-positive individuals and correlated with activation using mitogens in vitro. METHODS: Multiparameter flow cytometric analysis using combinations of CD3 CD28 and other markers was performed together with absolute cell counting in peripheral blood. Blast transformation and proliferative responses were also quantitated using the Cytoronabsolute after stimulation with phytohaemagglutinin (PHA) and anti-CD3. CD28- cells were also purified to confirm the observations. RESULTS: In HIV-1-negative individuals > 90% of CD3+ T cells were CD28+ and responded to stimulation, while CD3- CD16+ CD57+ NK-like cells were CD28- and failed to respond. In HIV-1-positive individuals the expression of CD28 was greatly reduced and the proportion of CD3+CD28- T cells expanded. CD8 lymphocytosis was caused entirely by the accumulation of CD28- T cells and many of these expressed activation markers human lymphocyte antigen-DR, CD38 and CD45RO on their membrane and molecules such as TIA-1 and perforin, associated with cytolytic function, in their cytoplasm. The strong positive correlation (r = 0.66) between the lack of CD28 expression and the poor proliferation from HIV-1-positive individuals was confirmed by demonstrating that only CD28+ cells transformed into lymphoblasts and proliferated. Although the CD28- including CD3+ T cells transiently expressed CD25 (interleukin-2R alpha), they did not undergo blastogenesis or activation measured by bromodeoxyuridine uptake and died after 3-4 days in culture. These observations were confirmed in costimulation experiments with anti-CD2 and anti-CD28. CONCLUSION: In HIV-1 infection activated CD3+CD28- T cells accumulate but are unresponsive to mitogens and anti-CD28. These cells appear to represent terminally differentiated effector cells which fail to respond to further stimuli because of the absence of a CD28 second signal.

Adult↗

PubMed1994

Co-stimulation with anti-CD28 (Kolt-2) enhances DNA synthesis by defective T cells in common variable immunodeficiency.

In normal T cells, an anti-CD28 MoAb (Kolt-2) will synergize with the mitogenic stimuli phytohaemagglutinin (PHA), anti-CD3 (OKT3) or a combination of anti-CD2 antibodies (OKT11 and GT2) in the induction of DNA synthesis. A subgroup of patients with common variable immunodeficiency (CVID) show a defect in DNA synthesis by T cells stimulated in vitro with the above mitogens. We have now investigated whether anti-CD28 will correct the defect. This strategy partially restored DNA synthesis, providing evidence that the CD28 co-stimulatory pathway in CVID T cells is normal. Ligation of CD28 acts through co-stimulating IL-2 secretion. The natural ligand (B7) for CD28 on antigen-presenting cells from CVID patients is expressed normally. We conclude that the defect in CVID T cells lies in pathways that lead to transcription of the IL-2 gene other than that induced by ligation of CD28 with Kolt-2.

Antibodies, Monoclonal↗

PubMed1994

Role of bcl-2 and apoptosis in viral infections.

For decades immunobiologists have focussed on lymphocyte activation leading to cellular proliferation as a means by which the numbers of antigen-responsive cells can be increased. The pendulum has now swung back and questions are being raised about the way in which the size of the activated lymphocyte populations, such as those found during certain acute viral infections, can be controlled. The process of apoptosis has gained widespread recognition as a physiological means by which unwanted cells can be removed. In this review we will discuss recent observations on the control of apoptosis in the activated and expanded T cell populations found during viral infections and how this process co-exists with certain mechanisms which enable the retention of the fittest cells which can participate in subsequent encounters with virus.

Animals↗

PubMed1993

The significance of low bcl-2 expression by CD45RO T cells in normal individuals and patients with acute viral infections. The role of apoptosis in T cell memory.

The bcl-2 gene product has been shown to prevent apoptotic cell death. We have now investigated the bcl-2 protein expression by resting and activated mature T cell populations. Freshly isolated CD45RO+ T cells within CD4+ and CD8+ subsets expressed significantly less bcl-2 than CD45RO- (CD45RA+) T cells (p < 0.001). When CD45RA+ T cells within both CD4+ and CD8+ subsets were activated in vitro, the transition to CD45RO phenotype was associated with a decrease in bcl-2 expression. Patients with acute viral infections such as infectious mononucleosis caused by Epstein-Barr virus infections or chickenpox, resulting from varicella zoster virus infection, had circulating populations of activated CD45RO+ T cells which also showed low bcl-2 expression. In these patients, a significant correlation was seen between low bcl-2 expression by activated T cells and their apoptosis in culture (r = 0.94, p < 0.001). These results suggest that the primary activation of T cells leads to the expansion of a population that is destined to perish unless rescued by some extrinsic event. Thus the suicide of CD45RO+ T cells could be prevented by the addition of interleukin 2 to the culture medium which resulted in a concomitant increase in the bcl-2 expression of these cells. Alternatively, apoptosis was also prevented by coculturing the activated T lymphocytes with fibroblasts, which maintained the viability of lymphoid cells in a restinglike state but with low bcl-2 expression. The paradox that the CD45RO+ population contains the primed/memory T cell pool yet expresses low bcl-2 and is susceptible to apoptosis can be reconciled by the observations that maintenance of T cell memory may be dependent on the continuous restimulation of T cells, which increases their bcl-2 expression. Furthermore, the propensity of CD45RO+ T cells to extravasate may facilitate encounter with fibroblast-like cells in tissue stroma and thus be an important additional factor which promotes the survival of selected primed/memory T cells in vivo.

Acquired Immunodeficiency Syndrome↗

PubMed1993

A possible role for bcl-2 in regulating T-cell memory--a 'balancing act' between cell death and survival.

One of the central questions in T-cell immunity is how the generation of T-cell memory may proceed simultaneously with the maintenance of T-cell homeostasis. In this article, we present a hypothesis that the regulation of bcl-2 gene expression within the activated CD45RO+ T-cell population may be one of the key factors in the balance between death and survival of these activated mature T cells, both in vitro and in vivo.

Animals↗

PubMed1993

Increased adherence of CD2 peripheral blood lymphocytes to cytomegalovirus-infected fibroblasts is blocked by anti-LFA-3 antibody.

In the accompanying manuscript (p. 405) we describe the up-regulation of the adhesion molecules LFA-3 and ICAM-1 on the surface of cytomegalovirus (CMV)-infected fibroblasts from days 1 to 5 post-infection. Peak expression was seen on day 2, when LFA-3 was twice, and ICAM-1 three times, the level on uninfected fibroblasts. The present study demonstrates a parallel increase in the adhesion of peripheral blood leucocytes to the CMV-infected fibroblasts, which was significantly greater than adhesion to uninfected fibroblasts from days 2 to 4 post-infection. This effect was seen from 2 to 24 hr of leucocyte-fibroblast co-culture. The increased binding to infected fibroblasts was accounted for by the CD2+ subset of lymphocytes. All subpopulations of CD2+ lymphocytes, namely CD3+, CD4+, and CD8+ cells, demonstrated increased adhesion to CMV-infected fibroblasts, suggesting that the CD2-LFA-3 interaction was an important component of the increased binding. This proposal was supported by the fact that the pretreatment of infected fibroblasts with monoclonal antibodies specific for LFA-3, significantly blocked the binding of CD2+ lymphocytes. Supernatants from infected fibroblasts, or co-cultures of leucocytes and infected fibroblasts, could transfer increased leucocyte binding to uninfected fibroblasts, suggesting that CMV might accentuate inflammatory responses. As lymphocytes can be activated by the CD2 pathway, CMV might also provoke nonspecific leucocyte responses to uninfected as well as infected cells, which could possibly contribute to tissue damage.

Antibodies, Monoclonal↗

PubMed1991

Human CD4+CD45R0+ and CD4+CD45RA+ T cells synergize in response to alloantigens.

Alloantigens, unlike recall antigens, activate both CD45RA+ (naive) and CD45R0+ (memory) CD4+ cells to the same extent. These T cell subsets may therefore interact with each other in response to alloantigens on transplanted grafts. We have investigated if the ability of activated CD4+CD45RA+ and CD4+CD45R0+ T cells to produce and respond to interleukin 2 (IL2) and IL4 may be involved in this interaction. After activation, both subsets up-regulate their IL2 receptor (IL2R) and IL4R expression, yet IL4 substantially enhanced the proliferation of the CD4+CD45RA+ but not of the CD4+CD45R0+ T cell subset, while IL2 increased the proliferation of CD4+CD45R0+ but not of the CD4+CD45RA+ T cells. Significantly, the CD4+CD45RA+ T cells synthesized two- to threefold more mRNA for IL2 than the CD4+CD45R0+ subset, while the CD4+CD45R0+ T cells synthesized mRNA for IL4 and interferon-gamma exclusively. The addition of IL2 to alloactivated CD4+CD45R0+ T cells further up-regulated their production of all three lymphokine mRNA; in contrast, IL4 induced an increase in mRNA for IL2 in only the alloactivated CD4+CD45RA+ subset. The reciprocity in the ability of both these CD4+ T cells to synthesize and respond to IL2 and IL4 may provide a rationale for the regulation of lymphokine interactions in vivo. Furthermore, the synergy between these subsets in response to alloantigens, which was directly quantitated by co-culturing CD4+CD45RA+ and CD4+CD45R0+ cells together prior to activation, may potentiate the alloreactivity against transplanted grafts in vivo.

Antigens, CD↗

PubMed1991

The synergy between naive and memory T cells during activation.

Naive and memory T-cell subsets differ in their ability to synthesize and respond to a variety of cytokines in vitro and each subset can produce cytokines that amplify the response of the other subset. The significance of these interactions to antigen responsiveness has, until now, been unclear. In this article Arne Akbar and colleagues point out that both subsets are activated to the same extent by alloantigen and suggest that synergy may be an important event in initiating potent responses against transplanted allografts.

Animals↗

PubMed1991

The effect of a chimeric mouse-human CD7 antibody on human T, natural killer, and lymphokine-activated killer cell activity in vitro.

A chimeric CD7 antibody has been constructed with mouse variable and human constant regions and is currently being assessed in the prophylaxis of renal graft rejection. In this study we have investigated if this antibody or its murine parental form inhibits the function of a number of immune effector mechanisms involved in host defense against infection and/or malignancy. Most memory T cells and all natural killer cells express the CD7 antigen and could therefore be affected by CD7 antibody. Murine and chimeric CD7 antibodies significantly inhibit the alloproliferation of naive (65 +/- 4% and 66 +/- 8%, respectively) but not memory T cells (86 +/- 2% and 98 +/- 4%, respectively) in a primary mixed lymphocyte reaction relative to the negative control CD10 antibody (P less than 0.001). The memory T cell proliferative response to recall antigen is also largely unaffected by murine and chimeric CD7 antibodies relative to the negative control antibody (91 +/- 12% and 103 +/- 10%, respectively). The CD7 antigen is almost completely modulated from the surface of NK cells after incubation for 24 hr with either the murine or chimeric CD7, but not the CD10, negative control. The modulation of CD7 antigen by antibody, however, does not affect the cytotoxic function of either the NK or lymphokine-activated killer cells significantly. Preincubation with the chimeric antibody however, consistently showed a small inhibition relative to the negative control of 75-80% in NK assays and to 80-90% in LAK assays. These data suggest that both murine and chimeric CD7 antibodies may have a selective effect on alloproliferation but may largely spare a major component of the host's innate immunity as well as memory T cell proliferation to previously encountered antigens.

Antibodies↗

PubMed1990

Inhibition of alloresponsive naive and memory T cells by CD7 and CD25 antibodies and by cyclosporine.

Human naive and memory T cells can be isolated from each other by their CD45RA and CD45RO expression, respectively. This enables the assessment of their differential sensitivities to immunosuppressive agents for the first time. We have investigated the ability of cyclosporine or CD7 and CD25 antibodies to selectively block alloantigen stimulated naive and memory T cells in vitro. CD7 antibodies blocked the proliferation of naive (P less than 0.025) but not memory T cells in a primary MLR. CD25 antibody inhibited both naive and memory subsets but a significantly greater effect was found on the memory T cells (P less than 0.005). The constitutive CD7 and CD25 antigen expression on resting naive or memory T cells was related to the inhibitory activities of these antibodies on both subsets. Accordingly, naive T cells expressed more CD7 antigen than memory cells while memory T cells displayed low levels of CD25 antigen that was absent from naive populations before activation. Cyclosporine, like CD25 antibody, inhibited both subsets in a primary MLR but had a greater effect on memory cells (P less than 0.02). Memory T cells, therefore, are more dependent than naive cells on IL-2 for proliferation. There was great individual variation in the ability of CsA to block the MLR. The simultaneous addition of CD25 or CD7 antibody together with CsA, however, enhanced the MLR inhibition as the effects of all three were additive. This suggested interference by these agents at different points during T cell activation. Thus, in CsA sensitive individuals, one-tenth of the optimal CsA concentration together with CD25 antibody maintained maximum immunosuppression in vitro. These results demonstrate the possibility of using CD7 and CD25 antibodies for selective inhibition of naive or memory T cells and also the possibility of augmenting the inhibition of and reducing the CsA concentration required for clinically effective immunosuppression.

Antibodies, Monoclonal↗

PubMed1990

The development of primed/memory CD8+ lymphocytes in vitro and in rejecting kidneys after transplantation.

In normal individuals, 80 +/- 5% of circulating CD8+ T cells express CD45RA, and 20 +/- 7% of these cells express CD45R0 antigens. After activation, CD8+ cells expressing CD45RA decrease to 56-67% while those expressing CD45R0 increase to 38-67%. Although precursors of alloantigen-specific cytotoxic T cells were found in both CD8+, CD45RA+ and CD8+, CD45R0+ subsets, the specific effector cells were exclusively CD8+, CD45R0. Allospecific cytotoxic CD8+ clones were also entirely CD45R0+. A lectin-dependent cytotoxic (LDC) assay unmasked a hierarchy of killing after alloactivation which was CD8+, CD45R0+ greater than CD8+, CD45RA+ greater than CD4+, CD45R0+ greater than CD4+, CD45RA+. The phenotype of CD8+ T cells in rejecting kidneys was similar to in vitro alloantigen-activated CD8+, CD45R0+ cells and cytotoxic CD8+ clones. Firstly there was an increase in the relative proportions of CD45R0+ (60 +/- 8%) and a decrease in CD45RA+ (35 +/- 10%) CD8+ cells relative to circulating CD8+ subsets. In the rejecting grafts, 34 +/- 9% of the CD8+ cells were also DR+ indicative of recent activation. Furthermore, 16% (range 4-35%) of rejecting CD8+ cells were Ki67+ suggesting that these cells were proliferating. Finally, 17% (range 4-53%) of T cells in rejecting kidneys simultaneously expressed both CD45RA and CD45R0 markers. These results show that in vitro alloantigen-activated CD8+, CD45R0+ cells represent a primed/memory cytotoxic population. In addition, they provide indirect evidence that a proportion of CD8+ cells in rejecting kidneys were actively switching from a naive to a memory phenotype in vivo in a manner analogous to that in vitro.

Antibodies, Monoclonal↗

PubMed1989

Cellular events during memory T-cell activation in vitro: the UCHL1 (180,000 MW) determinant is newly synthesized after mitosis.

After T-cell activation, a switch of leucocyte common antigen (LCA) expression occurs which is detected by the disappearance of CD45R and the appearance of UCHL1 positivity. Upon activation of CD45R+ T cells by phytohaemagglutinin (PHA), the IL-2 receptor (IL-2R) develops at 15 hr and blast cells enter the S phase at 20 hr, while remaining UCHL1-. At 40 hr, mitotic cells continue to express CD45R determinants, but weak UCHL1 reactivity is now detectable on 15-20% of these cells. By inhibiting the separation of daughter cells with cytochalasin B, it can be demonstrated that the UCHL1 antigen is newly synthesized in the Golgi apparatus before insertion into the membrane after the first mitosis. At 65 hr after activation, 80% of proliferating cells are UCHL1+. T cells undergo a similar development during allogeneic activation, and specific alloresponsive cells, tested in secondary mixed lymphocyte reaction (MLR), become greatly enriched among the proliferating UCHL1+ cells. Conversely, after a primary MLR, residual CD45R+ T cells are unresponsive to the original stimulus but can still proliferate in response to unrelated alloantigens. These results clarify both the time-course of the acquisition of UCHL1 antigen during the proliferative cycle of activated T cells, and indicate the shift of antigen responsive T-cell populations from the CD45R+ to the UCHL1+ pool.

Cells, Cultured↗

PubMed1988

Loss of CD45R and gain of UCHL1 reactivity is a feature of primed T cells.

A group of mAb recognizing the 200- and/or 220-kDa determinants (CD45R) of the leukocyte common Ag such as 2H4, WR16, MD4.3, and SN130 cross-block each other showing that they recognize a closely related epitope. The antibody UCHL1 reacts with a 180-kDa determinant of the leukocyte common Ag and exhibits a reciprocal T subset distribution pattern to the CD45R group. Peripheral blood T cells were 40% positive for UCHL1 and 58 to 65% positive for the CD45R antibodies; less than 1% of cells stained for both. On activation of CD45R+,UCHL1- T cells by PHA, up to 40% of cells became positive for both CD45R and UCHL1 by day 3. By day 7, CD45R+,UCHL1- cells fell from 90 to less than 21% whereas UCHL1+,CD45R- cells rose from 2 to 93%. Conversely, PHA-stimulated UCHL1+,CD45R- cells remained UCHL1+,CD45R- during the 7 days in culture showing that phenotypic change was unidirectional from CD45R+ to UCHL1+. In primary allogeneic mixed lymphocyte reactions, activated CD45R+ T cells also showed a change to UCHL1+. When these cells were rechallenged by the original alloantigen, the UCHL1+ cells showed 7- to 20-fold greater proliferation than the CD45R+ cells on day 3 after rechallenge. The recovery of virtually all alloantigen induced secondary proliferative response in the UCHL1+,CD45R- T cell population suggests that UCHL1 identifies a primed population of T cells which may include memory cells.

Antibodies, Monoclonal↗

PubMed1987

HTLV-III neutralizing antibody development in transfusion-dependent seropositive patients with beta-thalassemia.

Sera collected in New York in 1984 from 77 patients with homozygous beta-thalassemia were assayed for antibodies to HTLV-III by ELISA and Western blot techniques. Eight (12%) of the 66 hypertransfused thalassemics were seropositive. Retrospective sera of these eight individuals were examined by radioimmune precipitation (RIP), and assays for neutralization of virus infectivity were performed. With seroconversion, antibodies to viral envelope proteins appeared first and were correlated with development of neutralizing antibody. Affinity purified gp120, the major envelope glycoprotein of HTLV-III, blocked viral infectivity and absorbed neutralizing antibody activity from a positive serum. Neutralizing antibody titers mirrored antibody titers to gp120 by RIP. Antibody to gp120 sometimes occurred in the absence of neutralizing antibody, although the reverse was not true. One thalassemia patient who exhibited antibody to gp120 for 3 yr post-seroconversion failed to develop neutralizing antibody, acquired the acquired immunodeficiency syndrome with central nervous system involvement and lymphoma, and subsequently died. In contrast, all other seropositive thalassemics possessed neutralizing antibodies, and were asymptomatic or exhibited only lymphadenopathy. These results indicate that gp120 elicits neutralizing antibodies in the course of natural infection with HTLV-III. The relationship seen here between neutralizing antibody and better clinical outcome needs to be verified by additional studies.

AIDS-Related Complex↗

PubMed1987

Modulation of the defective natural killer activity seen in thalassaemia major with desferrioxamine and alpha-interferon.

We previously observed that natural killer (NK) activity toward K562 cells is markedly depressed in patients with beta-thalassaemia major. Here we report that these patients also exhibit significantly decreased (P less than 0.005) NK cytotoxicity against human fibroblasts infected with herpes simplex virus-type 1 (HSV-1) and that the amount of alpha-interferon (alpha-IFN) generated during the latter assays is significantly less than normal (P less than 0.005). This decreased production of alpha-IFN may account in part for the decreased NK activity seen in the thalassaemia patients. On the other hand, the cytotoxicity of their mononuclear cells (MNC) toward both K562 cells and HSV-1-infected fibroblasts could be augmented to the same extent as that of normal MNC by preincubation with alpha-IFN suggesting that thalassaemia MNC are capable of responding to this lymphokine despite their reduced ability to produce it. Moreover, preincubation of thalassaemia MNC with desferrioxamine (DFO), an iron-chelating agent, consistently increased the lysis of K562 cells indicating that the transfusion-induced iron overload which these patients experience may also contribute to the defective NK function seen in this disease. We have now found that preincubation of such MNC with DFO has no effect upon production of alpha-IFN when the MNC are cocultured with either HSV-1-infected fibroblasts or K562 cells. Combining DFO and alpha-IFN resulted in an increase in the NK activity of both normal and thalassaemia MNC against the two targets which was greater than that with alpha-IFN alone. In fact, preincubation of thalassaemia cells with this combination increased their NK activity toward K562 targets to that of untreated normal cells. This was true when either unfractionated MNC or NK-enriched fractions were used as effector cells. These results suggest that DFO and alpha-IFN enhance NK activity by different mechanisms, both of which appear to be reversibly impaired in thalassaemia patients.

Cytotoxicity, Immunologic↗

PubMed1987

Transfusion-related immunologic abnormalities in beta-thalassemia major.

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Cytoplasm↗

PubMed1986

Decreased natural killer activity in thalassemia major: a possible consequence of iron overload.

We have investigated the natural killer (NK) activity of both fractionated (Percoll density gradient) and unfractionated mononuclear cells from patients with beta-thalassemia major who are iron overloaded as a consequence of chronic transfusion therapy. These patients were found to have significantly decreased NK activity against K562 targets at all effector:target ratios tested (p less than 0.001). Both splenectomized and nonsplenectomized patients had normal proportions of Leu-11b-staining (NK) cells. Since they had normal to elevated absolute white cell and lymphocyte counts, a change in the absolute number of NK cells could not account for the decreased killing. We also found that the decrease in NK activity was transfusion related (r = -0.603, p less than 0.005). To determine whether or not this decrease in NK activity could be related to iron overload, we preincubated patient effector cells with desferrioxamine (DFO) or 2,3-dihydroxybenzoic acid (DHB) for 6 hr before addition of K562 targets. Both of these iron-chelating agents consistently increased the NK activity of cells from thalassemia patients. DHB had the greater effect, being able to increase patient NK activity to virtually normal levels. On the other hand, preincubation of cells from normal controls with DHB caused only a slight increase in NK activity, while similar treatment with DFO had little or no effect. When target cells were preincubated with the chelating agents before addition of either normal or patient effector cells, no change in cytotoxicity was seen, demonstrating that the chelating agents act at the effector cell level. Furthermore, if the chelating agents were saturated with iron prior to preincubation with the effectors, no increase in the cytotoxicity of thalassemic NK cells was observed. These results indicate that thalassemia patients have a reversible, transfusion-related decrease in NK function which may arise as a consequence of iron overload.

Cell Separation↗