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Biomedical subjects

Rafi Ahmed

Publications and source records attributed to Rafi Ahmed.

At least 55 records · Page 3Linked to original sources
PubMed2004

Generation and maintenance of immunological memory.

The key feature of the adaptive immune response is its specificity and the ability to generate and maintain memory. Preexisting antibodies in the circulation and at the mucosa provide the first line of defense against re-infection by extracellular as well as intracellular pathogens. Memory T cells are an important second line of defense against intracellular pathogens, and in particular against microbes that can cause chronic or latent infection. In this article we will review our current understanding of the generation and maintenance of B cell and T cell memory.

Animals↗
PubMed2004

Persistent infection with ebola virus under conditions of partial immunity.

Ebola hemorrhagic fever in humans is associated with high mortality; however, some infected hosts clear the virus and recover. The mechanisms by which this occurs and the correlates of protective immunity are not well defined. Using a mouse model, we determined the role of the immune system in clearance of and protection against Ebola virus. All CD8 T-cell-deficient mice succumbed to subcutaneous infection and had high viral antigen titers in tissues, whereas mice deficient in B cells or CD4 T cells cleared infection and survived, suggesting that CD8 T cells, independent of CD4 T cells and antibodies, are critical to protection against subcutaneous Ebola virus infection. B-cell-deficient mice that survived the primary subcutaneous infection (vaccinated mice) transiently depleted or not depleted of CD4 T cells also survived lethal intraperitoneal rechallenge for >/==" BORDER="0">25 days. However, all vaccinated B-cell-deficient mice depleted of CD8 T cells had high viral antigen titers in tissues following intraperitoneal rechallenge and died within 6 days, suggesting that memory CD8 T cells by themselves can protect mice from early death. Surprisingly, vaccinated B-cell-deficient mice, after initially clearing the infection, were found to have viral antigens in tissues later (day 120 to 150 post-intraperitoneal infection). Furthermore, following intraperitoneal rechallenge, vaccinated B-cell-deficient mice that were transiently depleted of CD4 T cells had high levels of viral antigen in tissues earlier (days 50 to 70) than vaccinated undepleted mice. This demonstrates that under certain immunodeficiency conditions, Ebola virus can persist and that loss of primed CD4 T cells accelerates the course of persistent infections. These data show that CD8 T cells play an important role in protection against acute disease, while both CD4 T cells and antibodies are required for long-term protection, and they provide evidence of persistent infection by Ebola virus suggesting that under certain conditions of immunodeficiency a host can harbor virus for prolonged periods, potentially acting as a reservoir.

Acute Disease↗
PubMed2004

Enhanced expression of cell cycle regulatory genes in virus-specific memory CD8+ T cells.

Unlike naive CD8+ T cells, antigen-experienced memory CD8+ T cells persist over time due to their unique ability to homeostatically proliferate. It was hypothesized that memory cells might differentially regulate the expression of genes that control the cell cycle to facilitate homeostatic proliferation. To test this, the expression levels of 96 different cell cycle regulatory genes were compared between transgenic naive and memory CD8+ T cells that specifically recognize the GP33-41 epitope of lymphocytic choriomeningitis virus (LCMV). It was discovered that relative to naive cells, memory cells overexpress several important genes that control the transition between G(1) and S phase. Some of these genes include those encoding cyclins D3, D2, B1, C, and H, cyclin-dependent kinases (cdk's) 4 and 6, the cdk inhibitors p16, p15, and p18, and other genes involved in protein degradation and DNA replication. Importantly, these differences were observed both in total populations of LCMV-specific naive and memory CD8+ cells and in LCMV-specific CD8+ T-cell populations that were in the G(1) phase of the cell cycle only. In addition, the expression differences between naive and memory cells were exaggerated following antigenic stimulation. The fact that memory cells are precharged with several of the major factors that are necessary for the G(1)- to-S-phase transition suggests they may require a lower threshold of stimulation to enter the cell cycle.

Animals↗
PubMed2004

Complexity of CNC transcription factors as revealed by gene targeting of the Nrf3 locus.

Cap'n'collar (CNC) family basic leucine zipper transcription factors play crucial roles in the regulation of mammalian gene expression and development. To determine the in vivo function of the CNC protein Nrf3 (NF-E2-related factor 3), we generated mice deficient in this transcription factor. We performed targeted disruption of two Nrf3 exons coding for CNC homology, basic DNA-binding, and leucine zipper dimerization domains. Nrf3 null mice developed normally and revealed no obvious phenotypic differences compared to wild-type animals. Nrf3(-/-) mice were fertile, and gross anatomy as well as behavior appeared normal. The mice showed normal age progression and did not show any apparent additional phenotype during their life span. We observed no differences in various blood parameters and chemistry values. We infected wild-type and Nrf3(-/-) mice with acute lymphocytic choriomeningitis virus and found no differences in these animals with respect to their number of virus-specific CD8 and CD4 T cells as well as their B-lymphocyte response. To determine whether the mild phenotype of Nrf3 null animals is due to functional redundancy, we generated mice deficient in multiple CNC factors. Contrary to our expectations, an absence of Nrf3 does not seem to cause additional lethality in compound Nrf3(-/-)/Nrf2(-/-) and Nrf3(-/-)/p45(-/-) mice. We hypothesize that the role of Nrf3 in vivo may become apparent only after appropriate challenge to the mice.

Animals↗
PubMed2004

Reflections on CD8 T-cell activation and memory.

CD8 T cells contribute to clearance and long-term protection following acute infection with certain viruses, bacteria, and protozoa, and may play an important role in tumor immunity. Primary adaptive CD8 T-cell responses have been conceptually divided into four phases: activation, expansion, contraction, and memory. We summarize each phase of the response, and discuss recent advances in our understanding of the development and maintenance of CD8 T-cell memory.

Animals↗
PubMed2003

CD43 modulates severity and onset of experimental autoimmune encephalomyelitis.

Experimental autoimmune encephalomyelitis (EAE) is a mouse model of multiple sclerosis characterized by infiltration of activated CD4(+) T lymphocytes into tissues of the CNS. This study investigated the role of CD43 in the induction and progression of EAE. Results demonstrate that CD43-deficient mice have reduced and delayed clinical and histological disease severity relative to CD43(+/+) mice. This reduction was characterized by decreased CD4(+) T cell infiltration of the CNS of CD43(-/-) mice but similar numbers of Ag-specific T cells in the periphery, suggesting a defect in T cell trafficking to the CNS. The absence of CD43 also affected cytokine production, as myelin oligodendrocyte glycoprotein (MOG) 35-55-specific CD43(-/-) CD4(+) T cells exhibited reduced IFN-gamma and increased IL-4 production. CD43(-/-) CD4(+) MOG-primed T cells exhibited reduced encephalitogenicity relative to CD43(+/+) cells upon adoptive transfer into naive recipients. These results suggest a role for CD43 in the differentiation and migration of MOG(35-55)-specific T cells in EAE, and identify it as a potential target for therapeutic intervention.

Adjuvants, Immunologic↗
PubMed2003

The rescaling method for quantifying the turnover of cell populations.

The dynamic nature of immune responses requires the development of appropriate experimental and theoretical tools to quantitatively estimate the division and death rates which determine the turnover of immune cells. A number of papers have used experimental data from BrdU and D-glucose labels together with a simple random birth-death model to quantify the turnover of immune cells focusing on HIV/SIV infections [Mohri et al. 279 (1998) 1223-1227, Hellerstein et al. 5 (1999) 83-89, Bonhoeffer et al. 164 (2000) 5049-5054, Mohri et al. 87 (2001) 1277-1287]. We show how uncertainties in the assumptions of the random birth-death model may lead to substantial errors in the parameters estimated. We then show how more accurate estimates can be obtained from the more recent CFSE data which allow to track the number of divisions each cell has undergone. Specifically, we: (i) describe a general stage-structured model of cell division where the probabilities of division and death are functions of time since the previous division; (ii) develop a rescaling method to identify invariant parameters (i.e. the ones that are independent of the specific functions describing division and death); (iii) show how these invariant parameters can be estimated, and (iv) illustrate this technique by applying it to CFSE data taken from the literature.

Adoptive Transfer↗
PubMed2003

Selective expression of the interleukin 7 receptor identifies effector CD8 T cells that give rise to long-lived memory cells.

A major unanswered question is what distinguishes the majority of activated CD8 T cells that die after an acute viral infection from the small fraction (5-10%) that survive to become long-lived memory cells. In this study we show that increased expression of the interleukin 7 receptor alpha-chain (IL-7Ralpha) identifies the effector CD8 T cells that will differentiate into memory cells. IL-7R(hi) effector cells contained increased amounts of antiapoptotic molecules, and adoptive transfer of IL-7R(hi) and IL-7R(lo) effector cells showed that IL-7R(hi) cells preferentially gave rise to memory cells that could persist and confer protective immunity. Thus, selective expression of IL-7R identifies memory cell precursors, and this marker may be useful in predicting the number of memory T cells generated after infection or immunization.

Adoptive Transfer↗
PubMed2003

Cutting edge: long-term B cell memory in humans after smallpox vaccination.

Memory B cells are a central component of humoral immunity, and yet little is known about their longevity in humans. Immune memory after smallpox vaccination (DryVax) is a valuable benchmark for understanding the longevity of B cell memory in the absence of re-exposure to Ag. In this study, we demonstrate that smallpox vaccine-specific memory B cells last for >50 years in immunized individuals. Virus-specific memory B cells initially declined postimmunization, but then reached a plateau approximately 10-fold lower than peak and were stably maintained for >50 years after vaccination at a frequency of approximately 0.1% of total circulating IgG(+) B cells. These persisting memory B cells were functional and able to mount a robust anamnestic Ab response upon revaccination. Additionally, virus-specific CD4(+) T cells were detected decades after vaccination. These data show that immunological memory to DryVax vaccine is long-lived and may contribute to protection against smallpox.

Adolescent↗
PubMed2003

Changing immunodominance patterns in antiviral CD8 T-cell responses after loss of epitope presentation or chronic antigenic stimulation.

The H-2(b)-restricted CD8 T-cell response against lymphocytic choriomeningitis virus is directed against at least 10 dominant and subdominant epitopes, including two newly identified epitopes in the nucleoprotein. We have used this set of epitopes to characterize the plasticity of the hierarchy under different experimental circumstances, i.e., loss of MHC class I molecules, loss of specific epitopes (CTL escape), and prolonged antigenic stimulation (chronic infection). We found that loss of epitope-specific responses was almost inevitably associated with compensatory responses against other, subdominant, epitopes. Multiple epitope loss was required to change the hierarchy. Persistent viral infection was associated with a loss of not only the dominant response against the NP396 epitope, but also a loss of subdominant responses against nucleoprotein epitopes. In contrast, responses against glycoprotein epitopes, dominant and subdominant, survived under chronic infection conditions, and even dominated the response (GP118). Our results suggest that the fate of each specific T-cell response during chronic infection is in part determined by the origin of the cognate epitopes, i.e, the proteins from which they are processed, or, more specifically, nucleoprotein versus glycoprotein. A model in which recruitment time plays a role in the longevity of antiviral T-cell responses during persistent infection is discussed.

Acute Disease↗
PubMed2003

Protection from lethal infection is determined by innate immune responses in a mouse model of Ebola virus infection.

A mouse-adapted strain of Ebola Zaire virus produces a fatal infection when BALB/cj mice are infected intraperitoneally (ip) but subcutaneous (sc) infection with the same virus fails to produce illness and confers long-term protection from lethal ip rechallenge. To identify immune correlates of protection in this model, we compared viral replication and cytokine/chemokine responses to Ebola virus in mice infected ip (10 PFU/mouse), or sc (100 PFU/mouse) and sc "immune" mice rechallenged ip (10(6) PFU/mouse) at several time points postinfection (pi). Ebola viral antigens were detected in the serum, liver, spleen, and kidneys of ip-infected mice by day 2 pi, increasing up to day 6. Sc-infected mice and immune mice rechallenged ip had no detectable viral antigens until day 6 pi, when low levels of viral antigens were detected in the livers of sc-infected mice only. TNF-alpha and MCP-1 were detected earlier and at significantly higher levels in the serum and tissues of ip-infected mice than in sc-infected or immune mice challenged ip. In contrast, high levels of IFN-alpha and IFN-gamma were found in tissues within 2 days after challenge in sc-infected and immune mice but not in ip-infected mice. Mice became resistant to ip challenge within 48 h of sc infection, coinciding with the rise in tissue IFN-alpha levels. In this model of Ebola virus infection, the nonlethal sc route of infection is associated with an attenuated inflammatory response and early production of antiviral cytokines, particularly IFN-alpha, as compared with lethal ip infection.

Animals↗
PubMed2003

Cutting edge: rapid in vivo killing by memory CD8 T cells.

In this study, we examined the cytotoxic activity of effector and memory CD8 T cells in vivo. At the peak of the CTL response following an acute lymphocytic choriomeningitis virus infection, effector CD8 T cells exhibited extremely rapid killing and started to eliminate adoptively transferred target cells within 15 min by a perforin-dependent mechanism. Although resting memory CD8 T cells are poorly cytolytic by in vitro (51)Cr release assays, there was rapid elimination (within 1-4 h) of target cells after transfer into immune mice, and both CD62L(high) and CD62L(low) memory CD8 T cells were able to kill rapidly in vivo. Strikingly, when directly compared on a per cell basis, memory CD8 T cells were only slightly slower than effector cells in eliminating target cells. These data indicate that virus specific memory CD8 T cells can rapidly acquire cytotoxic function upon re-exposure to Ag and are much more efficient killers in vivo than previously appreciated.

Adoptive Transfer↗
PubMed2003

TCR signal transduction in antigen-specific memory CD8 T cells.

Memory T cells are more responsive to Ag than naive cells. To determine whether memory T cells also have more efficient TCR signaling, we compared naive, effector, and memory CD8 T cells of the same antigenic specificity. Surprisingly, initial CD3 signaling events are indistinguishable. However, memory T cells have more extensive lipid rafts with higher phosphoprotein content before TCR engagement. Upon activation in vivo, they more efficiently induce phosphorylation of-LAT (linker for activation of T cells), ERK (extracellular signal-regulated kinase), JNK (c-Jun N-terminal kinase), and p38. Thus, memory CD8 T cells do not increase their TCR sensitivity, but are better poised to augment downstream signals. We propose that this regulatory mechanism might increase signal transduction in memory T cells, while limiting TCR cross-reactivity and autoimmunity.

Animals↗
PubMed2003

Mitochondrial potential and reactive oxygen intermediates in antigen-specific CD8+ T cells during viral infection.

Following many viral infections, there are large expansions of Ag-specific CD8(+) T cells. After viral clearance, mechanisms exist to ensure that the vast majority of effector cells undergo apoptosis. In studies of thymocyte apoptosis, loss of mitochondrial potential (deltapsi(m)) and excess production of reactive oxygen intermediates have been implicated as key events in cellular apoptosis. The purpose of the experiments presented in this work was to determine these parameters in Ag-specific CD8(+) T cells during a physiological response such as viral infection. Using lymphocytic choriomeningitis virus infection of mice, we found that Ag-specific CD8(+) effector T cells that had undergone recent TCR stimulation had an increased deltapsi(m). These cells also had increased levels of superoxide. As these cells progressed through the contraction of the immune response, their potential decreased, but superoxide levels remained similar to naive cells. One of the consequences of reduced mitochondrial potential is membrane permeability and subsequent caspase activation. We examined both the enzymatic activity and levels of cleaved caspase 3, an effector caspase, and could only detect increased levels in Ag-specific CD8(+) T cells on day 5 postinfection, a time point in which virus was still present. This contrasts with Ag-specific effector cells examined during the contraction phase that had no detectable caspase activity directly ex vivo. These data suggest that the apoptotic program begins earlier than previously expected on day 5, during the expansion phase.

Animals↗
PubMed2003

Models of CD8+ responses: 1. What is the antigen-independent proliferation program.

Recent experimental results show that even brief stimulation with antigen can cause antigen-specific CD8 T-cells to undergo sustained proliferation followed by differentiation into memory cells. These results show that the dynamics of these immune responses are not governed by constant monitoring of antigen levels, but rather that following stimulation immune cells commit to a "program". At present relatively little is known about the program which governs CD8 cell proliferation and differentiation. For example, we do not know whether the program is completely specified by the initial encounter of a T cell with antigen, or whether it subsequently can be modified by the amount of antigen present. Nor do we know whether the entire program for T cell proliferation and differentiation resides within the T cell itself, or whether some component(s) of the program are determined by cells or molecules external to the CD8 cell. In this paper we construct simple mathematical models which incorporate antigen-independent proliferation and differentiation of CD8 cells during acute infections. We use these models to determine what characteristics the program must have in order to be consistent with the existing data on the dynamics of CD8 responses, and in particular to answer the questions posed above. Our results suggest that the program is not completely defined by the initial encounter of T cell with antigen but may be augmented by exposure to antigen in a brief window shortly after infection; furthermore, parts of the program may reside external to the T-cells. Finally we examine some of the consequences of the "program" for pathogen-host coevolution.

Animals↗
PubMed2003

Therapeutic use of IL-2 to enhance antiviral T-cell responses in vivo.

Interleukin (IL)-2 is currently used to enhance T-cell immunity but can have both positive and negative effects on T cells. To determine whether these opposing results are due to IL-2 acting differently on T cells depending on their stage of differentiation, we examined the effects of IL-2 therapy during the expansion, contraction and memory phases of the T-cell response in lymphocytic choriomeningitis virus (LCMV)-infected mice. IL-2 treatment during the expansion phase was detrimental to the survival of rapidly dividing effector T cells. In contrast, IL-2 therapy was highly beneficial during the death phase, resulting in increased proliferation and survival of virus-specific T cells. IL-2 treatment also increased proliferation of resting memory T cells in mice that controlled the infection. Virus-specific T cells in chronically infected mice also responded to IL-2 resulting in decreased viral burden. Thus, timing of IL-2 administration and differentiation status of the T cell are critical parameters in designing IL-2 therapies.

Animals↗