Cytomegalovirus infection induces the accumulation of short-lived, multifunctional CD4+CD45RA+CD27+ T cells: the potential involvement of interleukin-7 in this process

The relative roles that ageing and lifelong cytomegalovirus (CMV) infection have in shaping naive and memory CD4⁺ T-cell repertoires in healthy older people is unclear. Using multiple linear regression analysis we found that age itself is a stronger predictor than CMV seropositivity for the decrease in CD45RA⁺ CD27⁺ CD4⁺ T cells over time. In contrast, the increase in CD45RA⁻ CD27⁻ and CD45RA⁺ CD27⁻ CD4⁺ T cells is almost exclusively the result of CMV seropositivity, with age alone having no significant effect. Furthermore, the majority of the CD45RA⁻ CD27⁻ and CD45RA⁺ CD27⁻ CD4⁺ T cells in CMV-seropositive donors are specific for this virus. CD45RA⁺ CD27⁻ CD4⁺ T cells have significantly reduced CD28, interleukin-7 receptor α (IL-7Rα) and Bcl-2 expression, Akt (ser473) phosphorylation and reduced ability to survive after T-cell receptor activation compared with the other T-cell subsets in the same donors. Despite this, the CD45RA⁺ CD27⁻ subset is as multifunctional as the CD45RA⁻ CD27⁺ and CD45RA⁻ CD27⁻ CD4⁺ T-cell subsets, indicating that they are not an exhausted population. In addition, CD45RA⁺ CD27⁻ CD4⁺ T cells have cytotoxic potential as they express high levels of granzyme B and perforin. CD4⁺ memory T cells re-expressing CD45RA can be generated from the CD45RA⁻ CD27⁺ population by the addition of IL-7 and during this process these cells down-regulated expression of IL-7R and Bcl-2 and so resemble their counterparts in vivo. Finally we showed that the proportion of CD45RA⁺ CD27⁻ CD4⁺ T cells of multiple specificities was significantly higher in the bone marrow than the blood of the same individuals, suggesting that this may be a site where these cells are generated.     

CD8+ CD28− T cells in vertically HIV-infected children

To evaluate whether vertical HIV infection interferes with the expression of CD28 on T lymphocytes, 25 HIV-infected children and 29 seroreverted children born to HIV⁺ mothers were studied. The percentage of CD28⁻ cells among CD8⁺ T lymphocytes was higher in HIV-infected children than in controls (P < 0.001). In fact, in HIV-infected children, this percentage was elevated from the first year of life, while in healthy seroreverted children, the proportion of CD28⁻ cells among CD8⁺ cells rose progressively with age (r = 0.49; P = 0.008). In HIV⁺ children, the CD8⁺ CD28⁻, but not CD8⁺ CD28⁺ cell proportion was significantly correlated with immunological markers of disease progression, such as CD4⁺ cell loss (r = −0.65; P < 0.001) and the level of in vitro spontaneous lymphocyte apoptosis (r = 0.53; P = 0.03).     

Role of γδ T lymphocytes in the development of Behçet's disease

Phenotypic and functional properties of γδ T cells, which play an important role in mucocutaneous immunity, were examined to elucidate whether immunological abnormality in Behc¸et's disease may be related to a specific T cell population. We found that CD45RA⁺Vγ9⁺Vδ2⁺γδ T cells, which constitute a minor population of γδ T cells in healthy individuals, were increased in number in Behc¸et's disease irrespective of disease activity. This CD45RA⁺ subset of γδ T cells in the active, but not inactive, phase of this disease expressed IL-2Rβ and HLA-DR, suggesting that they are activated in vivo in active Behc¸et's disease. In addition, the CD45RA⁺γδ T cells produced extreme amounts of tumour necrosis factor and contained perforin granules. These data indicate that a phenotypically distinct subset of γδ T cells, CD45RA⁺CD45RO⁻Vγ9⁺Vδ2⁺, may contribute to immunological abnormalities which may lead to complexity of pathophysiology in Behc¸et's disease.     

Low expression of CD39+/CD45RA+ on regulatory T cells (Treg) cells in type 1 diabetic children in contrast to high expression of CD101+/CD129+ on Treg cells in children with coeliac disease

Type 1 diabetes (T1D) and coeliac disease are both characterized by an autoimmune feature. As T1D and coeliac disease share the same risk genes, patients risk subsequently developing the other disease. This study aimed to investigate the expression of T helper (Th), T cytotoxic (Tc) and regulatory T cells (T_reg) in T1D and/or coeliac disease children in comparison to healthy children. Subgroups of T cells (Th : CD4⁺ or Tc : CD8⁺); naive (CD27⁺CD28⁺CD45RA⁺CCR7⁺), central memory (CD27⁺CD28⁺CD45RA⁻CCR7⁺), effector memory (early differentiated; CD27⁺CD28⁺CD45RA⁻CCR7⁻ and late differentiated; CD27⁻CD28⁻CD45RA⁻CCR7⁻), terminally differentiated effector cells (TEMRA; CD27⁻CD28⁻CD45RA⁺CCR7⁻) and T_reg (CD4⁺CD25⁺FOXP3⁺CD127⁻) cells, and their expression of CD39, CD45RA, CD101 and CD129, were studied by flow cytometry in T1D and/or coeliac disease children or without any of these diseases (reference group). Children diagnosed with both T1D and coeliac disease showed a higher percentage of TEMRA CD4⁺ cells (P < 0·05), but lower percentages of both early and late effector memory CD8⁺ cells (P < 0·05) compared to references. Children with exclusively T1D had lower median fluorescence intensity (MFI) of forkhead box protein 3 (FoxP3) (P < 0·05) and also a lower percentage of CD39⁺ and CD45RA⁺ within the T_reg population (CD4⁺CD25⁺FOXP3⁺CD127⁻) (P < 0·05). Children with exclusively coeliac disease had a higher MFI of CD101 (P < 0·01), as well as a higher percentage of CD129⁺ (P < 0·05), in the CD4⁺CD25^hi lymphocyte population, compared to references. In conclusion, children with combined T1D and coeliac disease have a higher percentage of differentiated CD4⁺ cells compared to CD8⁺ cells. T1D children show signs of low CD39⁺/CD45RA⁺ T_reg cells that may indicate loss of suppressive function. Conversely, children with coeliac disease show signs of CD101⁺/CD129⁺ T_reg cells that may indicate suppressor activity.     

Loss of the CD5+ and CD45RAhi B cell subsets in alcoholics

Chronic alcoholics are frequently immunodeficient, have polyclonal hypergammaglobulinaemia, and often have autoantibodies. Recent work in other diseases has shown that functional distinctions of possible relevance to autoimmunity and immunodeficiency can be found among the B cell subsets defined by differential expression of the surface markers CD5 and CD45RA. Therefore, we have evaluated the CD5,CD45RA B cell subsets of both chronic alcoholics without evidence of active liver disease (AWLD), and alcoholics admitted for acute alcoholic liver disease (ALD). Mean B cell numbers were normal in AWLD, but significantly reduced in ALD. Analysis of B cells by three-colour flow cytometry in 20 patients and 29 controls revealed a sharp decrease in the percentage of alcoholics’ B cells which were CD5⁺, 37·6% versus 16·3%, P<0·00001; absolute CD5⁺ B cell numbers were similarly reduced (58·9 cells/μl versus 20·9; P =0·0012). In addition to the loss of CD5⁺ B cells, there was a reduction in the percentage of B cells which are CD5⁻CD45RA^hi, leaving many patients with a B cell profile which was predominantly CD19⁺CD5⁻CD45RA^lo. This subset appears phenotypically similar to the IgM-producing CD5⁻CD45RA^lo subset described by others, and may be enriched for autoantibody-producing cells. One outlier patient was an ALD with 61% of B cells which were CD5⁺, which also is a profile consistent with increased autoantibody production.     

Anomalies of the CD8+ T cell pool in haemochromatosis: HLA-A3-linked expansions of CD8+CD28− T cells

The present study consists of a phenotypic and functional characterization of peripheral blood T lymphocytes in a group of 21 patients with hereditary haemochromatosis (HH), an MHC class I-linked genetic disease resulting in iron overload, and a group of 30 healthy individuals, both HLA-phenotyped. The HH patients studied showed an increased percentage of CD8⁺ CD28⁻ T cells with a corresponding reduction in the percentage of CD8⁺ CD28⁺ T cells in peripheral blood relative to healthy blood donors. No anomalies of CD28 expression were found in the CD4⁺ subset. The presence of the HLA-A3 antigen but not age accounted for these imbalances. Thus, an apparent failure of the CD8⁺ CD28⁺ T cell population ‘to expand’, coinciding with an ‘expansion’ of CD8⁺ CD28⁻ T cells in peripheral blood of HLA-A3⁺ but not HLA-A3⁻ HH patients was observed when compared with the respective HLA-A3-matched control group. A significantly higher percentage of HLA-DR⁺ but not CD45RO⁺ cells was also found within the peripheral CD8⁺ T cell subset in HH patients relative to controls. Phytohaemagglutinin (PHA) stimulation of peripheral blood mononuclear cells (PBMC) for 5 days showed: (i) that CD8⁺ CD28⁺ T cells both in controls and HH were able to expand in vitro; (ii) that CD8⁺ CD28⁻ T cells decreased markedly after activation in controls but not in HH patients. Moreover, functional studies showed that CD8⁺ cytotoxic T lymphocytes (CTL) from HH patients exhibited a diminished cytotoxic activity (approx. two-fold) in standard ⁵¹Cr-release assays when compared with CD8⁺ CTL from healthy controls. The present results provide additional evidence for the existence of phenotypic and functional anomalies of the peripheral CD8⁺ T cell pool that may underlie the clinical heterogeneity of this iron overload disease. They are of particular relevance given the recent discovery of a novel mutated MHC class I-like gene in HH.     

Elevation of CD8+ CD11b+ Leu-8− T cells is associated with the humoral immunodeficiency in myeloma patients

Recurrent bacterial infections due to humoral immunodeficiency are an important cause of death in myeloma patients. Recent data indicate that CD8⁺ T lymphocytes and a reduction of T helper type 1 cells with disease progression may be involved in the regulation of polyclonal immunoglobulin secretion. In mixed lymphocyte cultures derived from peripheral blood mononuclear cells (PBMC) of 24 myeloma patients with reduced immunoglobulin serum levels we investigated the association of CD4⁺ and CD8⁺ T cell subsets and immunoglobulin-secreting B cells (ISC) upon mitogenic stimulation with pokeweed mitogen (PWM) and concanavalin A (Con A). In supernatants of cultured PBMC of myeloma patients the spontaneous secretion of the type 1 cytokine interferon-gamma was reduced. After PWM stimulation reduced numbers of polyclonal ISC were found in 79% of patients, and monoclonal ISC were observed in 12% of patients. After Con A stimulation, again formation of polyclonal ISC was reduced, but monoclonal ISC were found in 41% of patients. Elevation of monoclonal and reduction of polyclonal ISC after stimulation with Con A were associated with an increase of CD8⁺ CD11b⁺ Leu-8⁻ T cells (P < 0.05). We conclude that the elevated numbers of CD8⁺ CD11b⁺ Leu-8⁻ T cells play a role in the stimulation of monoclonal and suppression of polyclonal immunoglobulin secretion in myeloma patients.     

Interleukin-13 is produced by activated human CD45RA+ and CD45R0+ T cells: modulation by interleukin-4 and interleukin-12

Interleukin (IL)-13 is a cytokine originally identified as a product of activated T cells. Little is known, however, about IL-13 production by human T cells and its modulation by other cytokines. Here, we show that IL-13 is produced by activated human CD4⁺ and CD8⁺ CD45R0⁺ memory T cells and CD4⁺ and CD8⁺ CD45RA⁺ naive T cells. In contrast, IL-4, which shares many biological activities with IL-13, is only produced by CD45R0⁺ T cells following activation. Analysis of intracellular cytokine production by single CD45RA⁺ and CD45R0⁺ T cells indicated that IL-13 continued to be produced for more than 24 h after stimulation, whereas IL-4 could not be detected after 24 h. These data were confirmed by measurement of specific mRNA and suggest that IL-13, unlike IL-4, but like interferon-γ (IFN-γ), is a cytokine with long-lasting kinetics. The majority of human CD45R0⁺ T cells produced IL-4 and IL-13 simultaneously. In contrast, IFN-γ protein was generally not co-expressed with IL-4 or IL-13. IL-4 added to primary cultures of highly purified peripheral blood T cells activated by the combination of anti-CD3+anti-CD28 mAb enhanced IL-13 production by CD45RA⁺ and to a lesser extent by CD45R0⁺ T cells. Under these conditions, however, IL-12 inhibited IL-13 production by CD45RA⁺ T cells and to a lesser extent by CD45R0⁺ T cells in a dose-dependent fashion. These inhibiting effects were not related to enhanced IFN-γ production induced by IL-12, since IFN-γ by itself did not affect IL-13 production. Collectively, our data indicate that IL-13 is produced by peripheral blood T cells which also produce IL-4, but not IFN-γ, and by naive CD45RA⁺ T cells which, in contrast, fail to produce IL-4. These observations, together with the long-lasting production of IL-13, suggest that IL-13 may have IL-4-like functions in situations where T cell-derived IL-4 is still absent or where its production has already been down-regulated.     

CD38+ CD45RBlow CD4+ T cells: a population of T cells with immune regulatory activities in vitro

An antibody reactive with CD38 revealed both phenotypic and functional heterogeneity amongst CD45RB^low cells. Functional analysis of the CD38⁺ and CD38⁻ fractions showed that the latter contained T cells which responded to recall antigens and produced high levels of cytokine in response to polyclonal stimulation. In contrast, the CD38⁺ population failed to proliferate or to produce detectable levels of cytokines. Despite appearing unresponsive, the CD38⁺ population significantly inhibited anti-CD3-induced proliferation and cytokine secretion by the reciprocal CD38⁻ population. Immune suppression required stimulation through the TCR and was dependent on a physical interaction between regulatory and responding CD4⁺ populations. It did not involve killing of the responding T cells or secretion of IL-10 or TGF-β. Despite some similarities there is no direct correlation between the in vitro suppression characteristic of the CD38⁺ CD45RB^low subset and in vivo suppression which has been shown to be mediated by unseparated CD45RB^low CD4⁺ T cells. However, these results demonstrate that two functionally distinct subsets of T cells reside within the antigen-exposed or CD45RB^low CD4⁺ T cell population and are thus generated in vivo: (1) conventional memory T cells which proliferate and secrete cytokines in response to activation and (2) a population of regulatory T cells which inhibit T cell activation in vitro. Antibodies reactive with CD38 may provide a useful tool with which to study the role of these T cell subsets in the induction and regulation of the immune response.     

CD4+CD45RA+T cells from adults respond to recall antigens after CD28 ligation

The leukocyte common antigen isoforms CD45RA and CD45RO havelong been used to discriminate human naive and memory T cellsrespectively. This model was largely based on the observationthat CD45RO⁺ T cells respond preferentially to and show a higherfrequency of precursors specific for recall antigens. However,CD45RA⁺ T cells have more stringent requirements for stimulationand standard in vitro assays may favour CD45RO⁺ cells in thisrespect. We tested the hypothesis that CD45RAf T cells respondpoorly to in vitro stimulation with recall antigens becauseof inadequate stimulation rather than a lack of precursors.Limiting dilution analyses (LDA) for tetanus toxoid (lT)-specificT cells were performed in the presence or absence of exogenousantLCD28 antibody. Addition of antLCD28 yielded no proliferationin the absence of specific antigen. The precursor frequencyfor lT in the CD4⁺ CD45RO⁺ population was –1:4000, whilethe frequency of CD4⁺ CD45RA⁺ T cells specific for lT was 4-to >>20-fold lower. Addition of anti-CD28 antibody didnot significantly alter the apparent precursor frequency forCD45RA⁺ cells but yielded an enhancement of the value for CD45RA⁺cells by 3- to >>5-fold. No enhancement of antigen-specificproliferation by antLCD28 was observed with CD45RA⁺ T cellsderived from cord blood, although phytohemagglutinin responsesof these cells were amplified by CD28 antibody. These resultsindicate that conventional LDA underestimate the true precursorfrequency of antigen-specific cells within the adult CD45RA⁺population and support the possibility that a small number ofcells revert from a primed (CD45RO⁺) to an unprimed (CD45RA⁺)state. The majority of memory T cells, however, appear to residein the CD45RO⁺ population     

In vitro responses of human CD45R0brightRA− and CD45R0−RAbright T cell subsets and their relationship to memory and naive T cells

The cellular basis of immunological memory, particularly with respect to T cells is not understood. In humans, monoclonal antibodies to CD45 have been used to identify memory (CD45R0) and naive (CD45RA) T cells. However, this identification has been called into question by various studies which suggest that high molecular weight CD45 isoforms may be re-expressed by previously activated cells. In the present study, using cultures which supported responses of naive T cells, we examined the responses of purified CD45R0^brightRA^? or CD45R0^?-RA^bright T cell subsets. The former subset was found to respond preferentially to recall antigens with minimal responses apparent to neo-(or non-recall)-antigens. The inverse pattern was found for CD45R0^?RA^bright T cells, which converted to CD45R0^brightRA^? after stimulation with a neo-antigen. Moreover, the two populations of T cells exhibited distinct response kinetics with a faster response evident from the CD45R0^brightRA^?T cells compared to the CD45R0^?RA^bright subset. The poor responses of CD45R0^?RA^bright T cells to recall antigens compared to neo-antigens suggests that this putative naive population is specifically depleted of reactive T cells following an encounter with antigen. We propose that T cell priming results in the stimulation of many CD45R0^?RA^bright T cells with various T cell receptor specificities from which memory T cells are selected for survival. If re-expression of higher molecular weight isoforms does occur in humans in vivo, our results suggest that R0 expression would be retained (CD45R0⁺RA⁺). Alternatively, if primed CD45R0^?RA^bright T cells exist, they are not prevalent in peripheral blood and thus may be sequestered within lymphoid tissues. Our data support the view that in human peripheral blood, CD45R0^bright and CD45RA^bright expression identify memory and naive CD4⁺ T cells, respectively.     

Susceptibility to clinically manifest cyclosporine A (CsA)-induced autoimmune disease is associated with interferon-gamma (IFN-γ)-producing CD45RC+RT6− T helper cells

Lethally irradiated Lewis (LEW) rats reconstituted with syngeneic bone marrow and given CsA for a 4-week period, develop, upon withdrawal of CsA, a graft-versus-host-like disease, so-called CsA-induced autoimmunity (CsA-AI). This T cell-mediated autoimmune disease is thymus-dependent; it is generally held that this disease is a consequence of aberrant T cell recovery brought about by CsA. In this study we determined mononuclear cell subsets phenotypically by tri-colour flow cytometry. A strong decrease in recent thymic emigrants (Thy1.1⁺, TCR αβ⁺) was observed as a consequence of CsA treatment, eventually resulting in decreased absolute peripheral T cell numbers. In these rats no altered CD4:CD8 T cell ratio was observed before onset of CsA-AI; CD4⁺ and CD8⁺ cells consisted predominantly of monocytes (CD4^dim+, TCR αβ⁻) and natural killer cells (CD8⁺, TCR αβ⁻), respectively. LEW rats, x-irradiated, syngeneic bone marrow-reconstituted and treated with CsA, showed a marked and persistent, relative expansion of mature CD45RC⁺, RT6⁻ Th cells. In contrast, Brown-Norway rats treated in a similar fashion, or LEW rats subjected to either CsA treatment or x-irradiation, did not show a comparable expansion of mature CD45RC⁺, RT6⁻ Th cells, nor did these animals develop CsA-AI. The CD45RC⁺, RT6⁻ Th cells produced IL-2, and moreover constituted the only Th subset producing IFN-γ upon stimulation, and therefore were considered as Th1-like effector cells. These results are consistent with the view that a persistent preponderance of Th1 cells and not the mere presence of autoreactive cells determines whether or not clinically manifest CsA-AI will occur.     

Selective infection of CD4 effector memory T lymphocytes leads to preferential depletion of memory T lymphocytes in R5 HIV-1-infected humanized NOD/SCID/IL-2Rγ mice

To investigate the events leading to the depletion of CD4⁺ T lymphocytes during long-term infection of human immunodeficiency virus type 1 (HIV-1), we infected human CD34⁺ cells-transplanted NOD/SCID/IL-2Rγ^null mice with CXCR4-tropic and CCR5-tropic HIV-1. CXCR4-tropic HIV-1-infected mice were quickly depleted of CD4⁺ thymocytes and both CD45RA⁺ naïve and CD45RA⁻ memory CD4⁺ T lymphocytes, while CCR5-tropic HIV-1-infected mice were preferentially depleted of CD45RA⁻ memory CD4⁺ T lymphocytes. Staining of HIV-1 p24 antigen revealed that CCR5-tropic HIV-1 preferentially infected effector memory T lymphocytes (T_EM) rather than central memory T lymphocytes. In addition, the majority of p24⁺ cells in CCR5-tropic HIV-1-infected mice were activated and in cycling phase. Taken together, our findings indicate that productive infection mainly takes place in the activated T_EM in cycling phase and further suggest that the predominant infection in T_EM would lead to the depletion of memory CD4⁺ T lymphocytes in CCR5-tropic HIV-1-infected mice.     

Antigen-independent adhesion of CD4 CD45RA T cells from cord blood

Antigen-independent adhesion of resting adult CD4⁺ CD45RO⁺ T cells to B lymphocytes has been shown to be transient and can be down-regulated by CD4 major histocompatibility complex (MHC) class II molecule interactions. Conversely, adhesion of adult CD4+ CD45RA+ subpopulation to B cells is not regulated by ligands of CD4. We have investigated the regulation of adhesion of cord blood CD45RA⁺ CD4⁺ T lymphocytes. In contrast to adult CD45RA⁺ CD4⁺ T cells, cord blood CD45RA⁺ CD4⁺ T cells were strongly sensitive to the down-regulation of adhesion mediated by the CD4-HLA class II interaction, since adhesion to MHC class II(⁺) B cells was transient and inhibited by an anti-CD4 antibody. In addition, human immunodeficiency virus gpl60, synthetic gpl06-derived peptides encompassing a CD4 binding site inhibited conjugate formation between cord blood CD45RA⁺ CD4+ T cells and B cells. Following activation of the cord blood CD4 T cells by an anti-CD3 antibody, a conversion from a transient to a stable adhesion pattern of cord blood CD4 T cells to B cells occurred in 2 days. The reversal to a transient adhesion occurred at day 8 following anti-CD3 activation in correlation with a complete shift to a CD45RO phenotype of the cord blood CD4 T cells. These data suggest that CD4 T cell adhesion can be developmentally regulated.     

Ribavirin modulates the conversion of human CD4+ CD25− T cell to CD4+ CD25+ FOXP3+ T cell via suppressing interleukin‐10‐producing regulatory T cell

Because regulatory T (Treg) cells play an important role in modulating the immune system response against both endogenous and exogenous antigens, their control is critical to establish immunotherapy against autoimmune disorders, chronic viral infections and tumours. Ribavirin (RBV), an antiviral reagent used with interferon, is known to polarize the T helper (Th) 1/2 cell balance toward Th1 cells. Although the immunoregulatory mechanisms of RBV are not fully understood, it has been expected that RBV would affect T reg cells to modulate the Th1/2 cell balance. To confirm this hypothesis, we investigated whether RBV modulates the inhibitory activity of human peripheral CD4⁺ CD25⁺ CD127⁻ T cells in vitro. CD4⁺ CD25⁺ CD127⁻ T cells pre-incubated with RBV lose their ability to inhibit the proliferation of CD4⁺ CD25⁻ T cells. Expression of Forkhead box P3 (FOXP3) in CD4⁺ CD25⁻ T cells was down-modulated when they were incubated with CD4⁺ CD25⁺ CD127⁻ T cells pre-incubated with RBV without down-modulating CD45RO on their surface. In addition, transwell assays and cytokine-neutralizing assays revealed that this effect depended mainly on the inhibition of interleukin-10 (IL-10) produced from CD4⁺ CD25⁺ CD127⁻ T cells. These results indicated that RBV might inhibit the conversion of CD4⁺ CD25⁻ FOXP3⁻ naive T cells into CD4⁺ CD25⁺ FOXP3⁺ adaptive Treg cells by down-modulating the IL-10-producing Treg 1 cells to prevent these effector T cells from entering anergy and to maintain Th1 cell activity. Taken together, our findings suggest that RBV would be useful for both elimination of long-term viral infections such as hepatitis C virus infection and for up-regulation of tumour-specific cellular immune responses to prevent carcinogenesis, especially hepatocellular carcinoma.     

Hyper-reactivity of mouse CD45RA − T cells

Mouse CD4 T cells have been partitioned into CD45RA and CD45RA^? subpopulations by means ot the monoclonal antibody 14.8. The CD45RA^? subpopulation proliferated more actively and generated more interleukin-4 (IL-4) in response to stimulation with anti-CD3 antibody and phytohemaglutinin, and more IL-2 in response to anti-CD3. This subpopulation is therefore hyper-reactive to these polyclonal stimulators, but does not show the bias towards T helper type 2 activity that has been found in studies with other related CD45 isoforms. No evidence of suppression was obtained by comparing proliferation of CD45RA^? cells in the presence and absence of CD45RA cells. Thus mouse CD4 T cells behave in these respects similarly to those of man, as is evident in a brief review of the quiescence-activation-quiescence cycle in the two species.     

Distinct Contributions of CD4+ and CD8+ T Cells to Pathogenesis of Trypanosoma brucei Infection in the Context of Gamma Interferon and Interleukin-10

Although gamma interferon (IFN-γ) and interleukin-10 (IL-10) have been shown to be critically involved in the pathogenesis of African trypanosomiasis, the contributions to this disease of CD4⁺ and CD8⁺ T cells, the major potential producers of the two cytokines, are incompletely understood. Here we show that, in contrast to previous findings, IFN-γ was produced by CD4⁺, but not CD8⁺, T cells in mice infected with Trypanosoma brucei. Without any impairment in the secretion of IFN-γ, infected CD8^−/− mice survived significantly longer than infected wild-type mice, suggesting that CD8⁺ T cells mediated mortality in an IFN-γ-independent manner. The increased survival of infected CD8^−/− mice was significantly reduced in the absence of IL-10 signaling. Interestingly, IL-10 was also secreted mainly by CD4⁺ T cells. Strikingly, depletion of CD4⁺ T cells abrogated the prolonged survival of infected CD8^−/− mice, demonstrating that CD4⁺ T cells mediated protection. Infected wild-type mice and CD8^−/− mice depleted of CD4⁺ T cells had equal survival times, suggesting that the protection mediated by CD4⁺ T cells was counteracted by the detrimental effects of CD8⁺ T cells in infected wild-type mice. Interestingly, CD4⁺ T cells also mediated the mortality of infected mice in the absence of IL-10 signaling, probably via excessive secretion of IFN-γ. Finally, CD4⁺, but not CD8⁺, T cells were critically involved in the synthesis of IgG antibodies during T. brucei infections. Collectively, these results highlight distinct roles of CD4⁺ and CD8⁺ T cells in the context of IFN-γ and IL-10 during T. brucei infections.     

Rapid re-expression of CD45RC on rat CD4 T cells in vitro correlates with a change in function

Rat CD4⁺ T cells are divided phenotypically by the anti-CD45RC monoclonal antibody OX22 into subsets with contrasting functions. Stimulation of T cells in vitro is known to induce a change in isoform from CD45RC⁺ to CD45RC^?. We have investigated the in vitro conditions which promote a switch in isoform in the opposite direction. We observed that a majority of CD45RC^? CD4 T cells (> 90%) spontaneously re-expressed CD45RC during the first 1-3 days of culture in both the presence and absence of alloantigen. The T cells remained CD45RC⁺ when cultured for 7 days in serum-free growth medium. However, alloantigen-activated lymphocytes, expressing the interleukin-2 receptor (IL-2R), down-regulated CD45RC by day 4 and remained CD45RC^? during the course of the experiment. Using mixtures of allotype-marked CD45RC⁺ and CD45RC^?T cells, it was demonstrated that each subset showed comparable survival, IL-2R expression and time courses of activation in response to alloantigen. The repertoire of neither subset was, therefore, deficient in terms of allorecognition. The rapid re-expression of CD45RC in culture was accompanied by a change in function: CD45RC⁺ “converts”, obtained by overnight culture of CD45RC^? T cells, induced significantly higher graft-versus-host responses. Thus, the transition in culture from CD45RC^? to CD45RC⁺ reflects a major functional reprogramming of the cell and not a trivial modulation of a surface antigen.     

Human CD45RA+ and CD45R0+ T cells exhibit similar CD3/T cell receptor-mediated transmembrane signaling capacities but differ in response to co-stimulatory signals

CD45RA⁺ cells have been described to be less responsive to CD3/T cell receptor (TcR)-mediated activation than CD45R0⁺ T cells. To analyze the underlying mechanism of the differential responses we compared CD3/TcR-triggered tyrosine phosphorylation in the two subsets and studied the role of co-stimulatory signals provided either by accessory cells or pharmacologic activation of protein kinase C by phorbol ester. Stimulation of purified CD45RA⁺ and CD45R0⁺ T cells with CD3/TcR antibodies induced similar patterns and intensities of tyrosine phosphorylation in the two subsets, but no proliferation. If accessory cells were used as the source of co-stimulatory signals, strong expression of the 55-kDa chain of the interleukin-2 (IL-2) receptor (CD25), significant IL-2 production and vigorous proliferation were observed in CD45R0⁺ cells, whereas CD45RA⁺ cells responded weakly. However, when CD3/TcR-mediated triggering was combined with activation of protein kinase C by phorbol ester, CD45RA⁺ cells responded strongly. These data indicate that the transmembrane signaling capacity of the T cell receptor expressed by CD45RA⁺ and CD45R0⁺ cells is similar and, therefore, is presumably not responsible for the differential reactivities of the two subsets. It is more likely that co-stimulatory signals determine whether CD3/TcR-initiated activation results in strong or weak responses.     

Age-related accumulation of LFA-1high cells in a CD8+CD45RAhigh T cell population

The differential expression of CD45 isoforms has been suggested as a marker for stages of post-thymic T cell development, that is, CD45RA+CD45R0^? T cells and CD45RA^?CD45R0⁺ T cells are supposed to be virgin and memory cells respectively. Recently, several adhesion molecules have been shown to be up-regulated on the cell surface of memory T cells, and have been suggested to serve as a memory marker. In this study, we investigated the levels of LFA-1 expression on T cells in various subpopulations defined by CD45 isoform expression in donors of various ages. In CD4⁺ T cells, the proportion of LFA-1^high cells among CD45RA^highCD45R0-T cells remained low in all age groups and did not show significant accumulation with age. CD4⁺CD45RA^?CD45R0^highTcells expressed LFA-1 at a higher level than CD4⁺CD45RA^highCD45R0^? T cells. Thus, the currently prevailing view that CD45RA and CD45R0 can be markers for virgin and primed cells was consistent with LFA-1 expression in CD4⁺ T cell population. In CD8⁺ T cells, however, CD45RA^highCD45R0^? T cells consisted of two distinct subpopulations, LFA-1^low and LFA-1^high cells, whereas CD45RA^?CD45R0^high T cells were almost exclusively LFA-1^high When CD29 expression was examined in place of LFA-1 expression, similar results were obtained; CD45RA^high CD45R0^? T cells consisted of two distinct subpopulations, CD29-^{to low} and CD^29high cells, while CD45RA-CD45R0^high T cells were mostly CD29^high. The proportion of LFA-1^high cells in the CD8+CD45RA^high T cell subpopulation increased significantly as a function of age (r = 0.9, p < 0.001). It ranged from 8% in a 14-year-old donor to 94% in a 79-year-old donor. Furthermore, the proportion of CD8⁺CD45RA^highLFA-1^high cells in the CD8⁺ T cell population increased significantly as a function of age (r = 0.85, p < 0.001). On the other hand, the proportion of LEA-1^high cells in CD8⁺CD45RA^? T cell subpopulation exceeded 90% in most donors irrespective of age. These results indicate that the CD8⁺CD45RA^highCD45R0^? T cell subpopulation contains a considerable number of LFA-1^high cells and CD29^high cells, phenotypically similar to previously activated cells. Thus, in terms of LFA-1 and CD29 expressions, the simple scheme that CD45RA is a marker of virgin cells is not applicable to the CD8⁺ T cell population.