Antioxidant treatment reduces expansion and contraction of antigen-specific CD8+ T cells during primary but not secondary viral infection

During many viral infections, antigen-specific CD8(+) T cells undergo large-scale expansion. After viral clearance, the vast majority of effector CD8(+) T cells undergo apoptosis. Previous studies have implicated reactive oxygen intermediates (ROI) in lymphocyte apoptosis. The purpose of the experiments presented here was to determine the role of ROI in the expansion and contraction of CD8(+) T cells in vivo during a physiological response such as viral infection. Mice were infected with lymphocytic choriomeningitis virus (LCMV) and treated with Mn(III)tetrakis(4-benzoic acid)porphyrin chloride (MnTBAP), a metalloporphyrin-mimetic compound with superoxide dismutase activity, from days 0 to 8 postinfection. At the peak of CD8(+)-T-cell response, on day 8 postinfection, the numbers of antigen-specific cells were 10-fold lower in MnTBAP-treated mice than in control mice. From days 8 to 30, a contraction phase ensued where the numbers of antigen-specific CD8(+) T cells declined 25-fold in vehicle-treated mice compared to a 3.5-fold decrease in MnTBAP-treated mice. Differences in contraction appeared to be due to greater proliferation in drug-treated mice. By day 38, the numbers of antigen-specific CD8(+) memory T cells were equivalent for the two groups. The administration of MnTBAP during secondary viral infection had no effect on the expansion of antigen-specific CD8(+) secondary effector T cells. These data suggest that ROI production is critical for the massive expansion and contraction of antigen-specific CD8(+) T cells during primary, but not secondary, viral infection.     

Regulation of CD8+ T cells undergoing primary and secondary responses to infection in the same host

Naive Ag-specific CD8(+) T cells expand, contract, and become memory cells after infection and/or vaccination. Memory CD8(+) T cells provide faster, more effective secondary responses against repeated exposure to the same pathogen. Using an adoptive transfer system with low numbers of trackable nontransgenic memory CD8(+) T cells, we showed that secondary responses can be comprised of both primary (naive) and secondary (memory) CD8(+) T cells after bacterial (Listeria monocytogenes) and/or viral (lymphocytic choriomeningitis virus) infections. The level of memory CD8(+) T cells present at the time of infection inversely correlated with the magnitude of primary CD8(+) T cell responses against the same epitope but directly correlated with the level of protection against infection. However, similar numbers of Ag-specific CD8(+) T cells were found 8 days postinfection no matter how many memory cells were present at the time of infection. Rapid contraction of primary CD8(+) T cell responses was not influenced by the presence of memory CD8(+) T cells. However, contraction of secondary CD8(+) T cell responses was markedly prolonged compared with primary responses in the same host mice. This situation occurred in response to lymphocytic choriomeningitis virus or L. monocytogenes infection and for CD8(+) T cell responses against multiple epitopes. The delayed contraction of secondary CD8(+) T cells was also observed after immunization with peptide-coated dendritic cells. Together, the results show that the level of memory CD8(+) T cells influences protective immunity and activation of naive precursors specific for the same epitope but has little impact on the magnitude or program of the CD8(+) T cell response.     

Population dynamics of naive and memory CD8 T cell responses after antigen stimulations in vivo

The extent to which the progeny of one primary memory CD8 T cell differs from the progeny of one naive CD8 T cell of the same specificity remains an unresolved question. To explore cell-autonomous functional differences between naive and memory CD8 T cells that are not influenced by differences in the priming environment, an experimental model has been developed in which physiological numbers of both populations of cells were cotransferred into naive hosts before Ag stimulation. Interestingly, naive CD8 T cells undergo greater expansion in numbers than do primary memory CD8 T cells after various infections or immunizations. The intrinsic ability of one naive CD8 T cell to give rise to more effector CD8 T cells than one memory CD8 T cell is independent of the number and quality of primary memory CD8 T cells present in vivo. The sustained proliferation of newly activated naive CD8 T cells contributed to their greater magnitude of expansion. Additionally, longitudinal analyses of primary and secondary CD8 T cell responses revealed that on a per-cell basis naive CD8 T cells generate higher numbers of long-lived memory cells than do primary memory CD8 T cells. This enhanced "memory generation potential" of responding naive CD8 T cells occurred despite the delayed contraction of secondary CD8 T cell responses. Taken together, the data in this study revealed previously unappreciated differences between naive and memory CD8 T cells and will help further define the functional potential for both cell types.     

Stimulation by soluble CD70 promotes strong primary and secondary CD8+ cytotoxic T cell responses in vivo

Identification of the signals required for optimal differentiation of naive CD8(+) T cells into effector and memory cells is critical for the design of effective vaccines. In this study we demonstrate that CD27 stimulation by soluble CD70 considerably enhances the magnitude and quality of the CD8(+) T cell response. Stimulation with soluble CD70 in the presence of Ag significantly enhanced the proliferation of CD8(+) T cells and their ability to produce IL-2 and IFN-gamma in vitro. Administration of Ag and soluble CD70 resulted in a massive (>300-fold) expansion of Ag-specific CD8(+) T cells in vivo, which was due to the enhanced proliferation and survival of activated T cells. In mice that received Ag and soluble CD70, CD8(+) T cells developed into effectors with direct ex vivo cytotoxicity. Furthermore, unlike peptide immunization, which resulted in a diminished response after rechallenge, CD27 stimulation during the primary challenge evoked a strong secondary response upon rechallenge with the antigenic peptide. Thus, in addition to increasing the frequency of primed Ag-specific T cells, CD27 signaling during the primary response instills a program of differentiation that allows CD8(+) T cells to overcome a state of unresponsiveness. Taken together these results demonstrate that soluble CD70 has potent in vivo adjuvant effects for CD8(+) T cell responses.     

Naive, effector, and memory CD8 T cells in protection against pulmonary influenza virus infection: homing properties rather than initial frequencies are crucial.

The goal of adoptive immunotherapy is to target a high number of persisting effector cells to the site of a virus infection or tumor. In this study, we compared the protective value of hemagglutinin peptide-specific CD8 T cells generated from the clone-4 TCR-transgenic mice, defined by different stages of their differentiation, against lethal pulmonary influenza infection. We show that the adoptive transfer of high numbers of Ag-specific unprimed, naive CD8 T cells failed to clear the pulmonary virus titer and to promote host survival. The same numbers of in vitro generated primary Ag-specific Tc1 effector cells, producing high amounts of IFN-gamma, or resting Tc1 memory cells, generated from these effectors, were protective. Highly activated CD62Llow Tc1 effectors accumulated in the lung with rapid kinetics and most efficiently reduced the pulmonary viral titer early during infection. The resting CD62Lhigh naive and memory populations first increased in cell numbers in the draining lymph nodes. Subsequently, memory cells accumulated more rapidly and to a greater extent in the lung lavage as compared with naive cells. Thus, effector cells are most effective against a localized virus infection, which correlates with their ability to rapidly distribute at the infected tissue site. The finding that similar numbers of naive Ag-specific CD8 T cells are not protective supports the view that qualitative differences between the two resting populations, the naive and the memory population, may play a major role in their protective value against disease.     

Dynamics of human respiratory virus-specific CD8+ T cell responses in blood and airways during episodes of common cold

We determined the dynamics of CD8(+) T cells specific for influenza virus and respiratory syncytial virus in blood and tracheostoma aspirates of children during the course of respiratory infections. We showed that during localized respiratory infections the ratio of activated effector CD8(+) T cells to resting memory/naive CD8(+) T cells in peripheral blood increased significantly. Furthermore, the number of effector/memory T cells specific for respiratory viruses declined in blood and increased in the airways, suggesting that these T cells redistributed from blood to airways. T cells specific for the infecting virus were present in the airways for longer periods at increased levels than nonspecifically recruited bystander T cells. After clearance of the infection, the ratio of resting memory and naive CD8(+) T cells normalized in peripheral blood and also memory T cell numbers specific for unrelated viruses that declined during the infection due to bystander recruitment were restored. Taken together, these results showed a significant systemic T cell response during relatively mild secondary infections and extensive dynamics of virus-specific and nonspecific Ag-experienced T cells.     

Discriminating between different pathways of memory CD8+ T cell differentiation

Despite the rapid accumulation of quantitative data on the dynamics of CD8(+) T cell responses following acute viral or bacterial infections of mice, the pathways of differentiation of naive CD8(+) T cells into memory during an immune response remain controversial. Currently, three models have been proposed. In the "stem cell-associated differentiation" model, following activation, naive T cells differentiate into stem cell-like memory cells, which then convert into terminally differentiated short-lived effector cells. In the "linear differentiation" model, following activation, naive T cells first differentiate into effectors, and after Ag clearance, effectors convert into memory cells. Finally, in the "progressive differentiation" model, naive T cells differentiate into memory or effector cells depending on the amount of specific stimulation received, with weaker stimulation resulting in formation of memory cells. This study investigates whether the mathematical models formulated from these hypotheses are consistent with the data on the dynamics of the CD8(+) T cell response to lymphocytic choriomeningitis virus during acute infection of mice. Findings indicate that two models, the stem cell-associated differentiation model and the progressive differentiation model, in which differentiation of cells is strongly linked to the number of cell divisions, fail to describe the data at biologically reasonable parameter values. This work suggests additional experimental tests that may allow for further discrimination between different models of CD8(+) T cell differentiation in acute infections.     

Effector CD4 T cells are biochemically distinct from the memory subset: evidence for long-term persistence of effectors in vivo.

Memory T cell responses are believed to be mediated by long-lived memory T cells that arise directly from a subset of short-lived, activated effector T cells that have reverted to the resting state. Although widely accepted, definitive proof that memory T cells arise from effectors is lacking because of the inability to reliably distinguish these subsets based on known phenotypic or functional parameters. We have used a biochemical approach to distinguish effector and memory CD4 T cell subsets and follow the differentiative fate of effector cells in vivo. When examined biochemically, effector and memory CD4 T cells are strikingly distinct and exhibit qualitative and quantitative differences in tyrosine phosphorylation. These effector-specific patterns were identical in effectors derived either from naive CD4 T cells (primary effectors) or memory CD4 T cells (memory effectors). To monitor the fate of effector cells in vivo, Ag-activated CD4+ TCR-transgenic T cells were transferred into irradiated BALB/c mice. These TCR-transgenic CD4 T cells persisted in adoptive hosts for several months, gave a recall response to Ag, yet exhibited effector-specific biochemical profiles. These results suggest that a subset of effector CD4 T cells can persist in vivo and contribute to long-term immunity by mediating secondary immune responses.     

Cognate memory CD4+ T cells generated with dendritic cell priming influence the expansion, trafficking, and differentiation of secondary CD8+ T cells and enhance tumor control

CD4(+) T cells are known to provide support for the activation and expansion of primary CD8(+) T cells, their subsequent differentiation, and ultimately their survival as memory cells. However, the importance of cognate memory CD4(+) T cells in the expansion of memory CD8(+) T cells after re-exposure to Ag has been not been examined in detail. Using bone marrow-derived dendritic cells pulsed with cognate or noncognate MHC class I- and class II-restricted peptides, we examined whether the presence of memory CD4(+) T cells with the same Ag specificity as memory CD8(+) T cells influenced the quantity and quality of the secondary CD8(+) T cell response. After recombinant vaccinia virus-mediated challenge, we demonstrate that, although cognate memory CD4(+) T cells are not required for activation of secondary CD8(+) T cells, their presence enhances the expansion of cognate memory CD8(+) T cells. Cognate CD4(+) T cell help results in an approximate 2-fold increase in the frequency of secondary CD8(+) T cells in secondary lymphoid tissues, and can be accounted for by enhanced proliferation in the secondary CD8(+) T cell population. In addition, cognate memory CD4(+) T cells further selectively enhance secondary CD8(+) T cell infiltration of tumor-associated peripheral tissue, and this is accompanied by increased differentiation into effector phenotype within the secondary CD8(+) T cell population. The consequence of these improvements to the magnitude and phenotype of the secondary CD8(+) T cell response is substantial increase in control of tumor outgrowth.     

Stable CD8+ T cell memory during persistent Trypanosoma cruzi infection

CD8(+) T cell responses to persistent infections caused by intracellular pathogens are dominated by resting T effectors and T effector memory cells, with little evidence suggesting that a T central memory (T(CM)) population is generated. Using a model of Trypanosoma cruzi infection, we demonstrate that in contrast to the T effector/T effector memory phenotype of the majority of T. cruzi-specific CD8(+) T cells, a population of cells displaying hallmark characteristics of T(CM) cells is also present during long-term persistent infection. This population expressed the T(CM) marker CD127 and a subset expressed one or more of three other T(CM) markers: CD62L, CCR7, and CD122. Additionally, the majority of CD127(high) cells were KLRG1(low), indicating that they have not been repetitively activated through TCR stimulation. These CD127(high) cells were better maintained than their CD127(low) counterparts following transfer into naive mice, consistent with their observed surface expression of CD127 and CD122, which confer the ability to self-renew in response to IL-7 and IL-15. CD127(high) cells were capable of IFN-gamma production upon peptide restimulation and expanded in response to challenge infection, indicating that these cells are functionally responsive upon Ag re-encounter. These results are in contrast to what is typically observed during many persistent infections and indicate that a stable population of parasite-specific CD8(+) T cells capable of Ag-independent survival is maintained in mice despite the presence of persistent Ag.     

An in vivo cytotoxicity threshold for influenza A virus-specific effector and memory CD8(+) T cells

Influenza A virus infection of C57BL/6 (B6) mice is characterized by prominent CD8(+) T cell responses to H2D(b) complexed with peptides from the viral nucleoprotein (NP(366), ASNENMETM) and acid polymerase (PA(224), SSLENFRAYV). An in vivo cytotoxicity assay that depends on the adoptive transfer of peptide-pulsed, syngeneic targets was used in this study to quantitate the cytotoxic potential of D(b)NP(366)- and D(b)PA(224)-specific acute and memory CD8(+) T cells following primary or secondary virus challenge. Both T cell populations displayed equivalent levels of in vivo effector function when comparable numbers were transferred into naive B6 hosts. Cytotoxic activity following primary infection clearly correlated with the frequency of tetramer-stained CD8(+) T cells. This relationship looked, however, to be less direct following secondary exposure, partly because the numbers of CD8(+)D(b)NP(366)(+) T cells were greatly in excess. However, calculating the in vivo E:T ratios indicated that in vivo lysis, like many other biological functions, is threshold dependent. Furthermore, the capacity to eliminate peptide-pulsed targets was independent of the differentiation state (i.e., primary or secondary effectors) and was comparable for the two T cell specificities that were analyzed. These experiments provide insights that may be of value for adoptive immunotherapy, where careful consideration of both the activation state and the number of effector cells is required.     

Prolonged antigen expression following DNA vaccination impairs effector CD8+ T cell function and memory development

After priming, naive T cells undergo a program of expansion, contraction, and memory formation. Numerous studies have indicated that only a brief period of antigenic stimulation is required to fully commit CD8+ T cells to this program. Nonetheless, the persistence of Ag may modulate the eventual fate of CD8+ T cells. Using DNA delivery, we showed previously that direct presentation primes high levels of effector CD8+ T cells as compared with cross-presentation. One explanation now revealed is that prolonged cross-presentation limits effector cell expansion and function. To analyze this, we used a drug-responsive system to regulate Ag expression after DNA injection. Reducing expression to a single burst expanded greater numbers of peptide-specific effector CD8+ T cells than sustained Ag. Consequences for memory development were assessed after boosting and showed that, although persistent Ag maintained higher numbers of tetramer-positive CD8+ T cells, these expanded less (approximately 4-fold) than those induced by transient Ag expression (approximately 35-fold). Transient expression at priming therefore led to a net higher secondary response. In terms of vaccine design, we propose that the most effective DNA-based CD8+ T cell vaccines will be those that deliver a short burst of Ag.     

Functional expression of the chemokine receptor CCR5 on virus epitope-specific memory and effector CD8+ T cells

Because the chemokine receptor CCR5 is expressed on Th1 CD4(+) cells, it is important to investigate the expression and function of this receptor on other T cells involved in Th1 immune responses, such as Ag-specific CD8(+) T cells, which to date have been only partially characterized. Therefore, we analyzed the expression and function of CCR5 on virus-specific CD8+ T cells identified by HLA class I tetramers. Multicolor flow cytometry analysis demonstrated that CCR5 is expressed on memory (CD28+CD45RA-) and effector (CD28-CD45RA- and CD28-CD45RA+) CD8+ T cells but not on naive (CD28+CD45RA+) CD8+ T cells. CCR5 expression was much lower on two effector CD8+ T cells than on memory CD8+ T cells. Analysis of CCR7 and CCR5 expression on the different types of CD8+ T cells showed that memory CD8+ T cells have three phenotypic subsets, CCR5+CCR7-, CCR5+CCR7+, and CCR5-CCR7+, while naive and effector CD8+ T cells have CCR5-CCR7+ and CCR5+CCR7- phenotypes, respectively. These results suggest the following sequence for differentiation of memory CD8+ T cells: CCR5-CCR7+-->CCR5+CCR7+-->CCR5+CCR7-. CCR5+CD8+ T cells effectively migrated in response to RANTES, suggesting that CCR5 plays a critical role in the migration of Ag-specific effector and differentiated memory CD8+ T cells to inflammatory tissues and secondary lymphoid tissues. This is in contrast to CCR7, which functions as a homing receptor in migration of naive and memory CD8+ T cells to secondary lymphoid tissues.     

Differential regulation of primary and secondary CD8+ T cells in the central nervous system

T cell accumulation and effector function following CNS infection is limited by a paucity of Ag presentation and inhibitory factors characteristic of the CNS environment. Differential susceptibilities of primary and recall CD8+ T cell responses to the inhibitory CNS environment were monitored in naive and CD8+ T cell-immune mice challenged with a neurotropic coronavirus. Accelerated virus clearance and limited spread in immunized mice was associated with a rapid and increased CNS influx of virus-specific secondary CD8+ T cells. CNS-derived secondary CD8+ T cells exhibited increased cytolytic activity and IFN-gamma expression per cell compared with primary CD8+ T cells. However, both Ag-specific primary and secondary CD8+ T cells demonstrated similar contraction rates. Thus, CNS persistence of increased numbers of secondary CD8+ T cells reflected differences in the initial pool size during peak inflammation rather than enhanced survival. Unlike primary CD8+ T cells, persisting secondary CD8+ T cells retained ex vivo cytolytic activity and expressed high levels of IFN-gamma following Ag stimulation. However, both primary and secondary CD8+ T cells exhibited reduced capacity to produce TNF-alpha, differentiating them from effector memory T cells. Activation of primary and secondary CD8+ T cells in the same host using adoptive transfers confirmed similar survival, but enhanced and prolonged effector function of secondary CD8+ T cells in the CNS. These data suggest that an instructional program intrinsic to T cell differentiation, rather than Ag load or factors in the inflamed CNS, prominently regulate CD8+ T cell function.     

Intrinsic differences in the proliferation of naive and memory human B cells as a mechanism for enhanced secondary immune responses

Humoral immune responses elicited after secondary exposure to immunizing Ag are characterized by robust and elevated reactivity of memory B cells that exceed those of naive B cells during the primary response. The mechanism underlying this difference in responsiveness of naive vs memory B cells remains unclear. We have quantitated the response of naive and memory human B cells after in vitro stimulation with T cell-derived stimuli. In response to stimulation with CD40 ligand alone or with IL-10, both IgM-expressing and Ig isotype-switched memory B cells entered their first division 20-30 h earlier than did naive B cells. In contrast, the time spent traversing subsequent divisions was similar. Consistent with previous studies, only memory cells differentiated to CD38(+) blasts in a manner that increased with consecutive division number. These differentiated CD38(+) B cells divided faster than did CD38(-) memory B cell blasts. Proliferation of CD40 ligand-stimulated naive B cells as well as both CD38(+) and CD38(-) cells present in cultures of memory B cells was increased by IL-10. In contrast, IL-2 enhanced proliferation of CD38(-) and CD38(+) memory B cell blasts, but not naive cells. Thus, memory B cells possess an intrinsic advantage over naive B cells in both the time to initiate a response and in the division-based rate of effector cell development. These differences help explain the accelerated Ab response exhibited by memory B cells after secondary challenge by an invading pathogen, a hallmark of immunological memory.     

Gene expression in antigen-specific CD8+ T cells during viral infection

Following infection with intracellular pathogens, Ag-specific CD8(+) T cells become activated and begin to proliferate. As these cells become activated, they elaborate effector functions including cytokine production and cytolysis. After the infection has been cleared, the immune system returns to homeostasis through apoptosis of the majority of the Ag-specific effector cells. The surviving memory cells can persist for extended periods and provide protection against reinfection. Little is known about the changes in gene expression as Ag-specific cells progress through these stages of development, i.e., naive to effector to memory. Using recombinant MHC class I tetramers, we isolated Ag-specific CD8(+) T cells from mice infected with lymphocytic choriomeningitis virus at various time points and performed semiquantitative RT-PCR. We examined expression of: 1) genes involved in cell cycle control, 2) effector and regulatory functions, and 3) susceptibility to apoptosis. We found that Ag-specific CD8(+) memory T cells contain high steady-state levels of Bcl-2, BAX:, IFN-gamma, and lung Kruppel-like factor (LKLF), and decreased levels of p21 and p27 mRNA. Moreover, the pattern of gene expression between naive and memory cells is distinct and suggests that these two cell types control susceptibility to apoptosis through different mechanisms.     

Dynamic regulation of IFN-gamma signaling in antigen-specific CD8+ T cells responding to infection

IFN-gamma plays a critical role in the CD8(+) T cell response to infection, but when and if this cytokine directly signals CD8(+) T cells during an immune response is unknown. We show that naive Ag-specific CD8(+) T cells receive IFN-gamma signals within 12 h after in vivo infection with Listeria monocytogenes and then become unresponsive to IFN-gamma throughout the ensuing Ag-driven expansion phase. Ag-specific CD8(+) T cells regain partial IFN-gamma responsiveness throughout the contraction phase, whereas the memory pool exhibits uniform, but reduced, responsiveness that is also modulated during the secondary response. The responsiveness of Ag-specific CD8(+) T cells to IFN-gamma correlated with modulation in the expression of IFN-gammaR2, but not with IFN-gammaR1 or suppressor of cytokine signaling-1. This dynamic regulation suggests that early IFN-gamma signals participate in regulation of the primary CD8(+) T cell response program, but that evading or minimizing IFN-gamma signals during expansion and the memory phase may contribute to appropriate regulation of the CD8(+) T cell response.     

Alloreactive memory T cells are responsible for the persistence of graft-versus-host disease

Graft-vs-host disease (GVHD) is caused by a donor T cell anti-host reaction that evolves over several weeks to months, suggesting a requirement for persistent alloreactive T cells. Using the C3H.SW anti-C57BL/6 (B6) mouse model of human GVHD directed against minor histocompatibility Ags, we found that donor CD8(+) T cells secreting high levels of IFN-gamma in GVHD B6 mice receiving C3H.SW naive CD8(+) T cells peaked by day 14, declined by day 28 after transplantation, and persisted thereafter, corresponding to the kinetics of a memory T cell response. Donor CD8(+) T cells recovered on day 42 after allogeneic bone marrow transplantation expressed the phenotype of CD44(high)CD122(high)CD25(low), were able to homeostatically survive in response to IL-2, IL-7, and IL-15 and rapidly proliferated upon restimulation with host dendritic cells. Both allogeneic effector memory (CD44(high)CD62L(low)) and central memory (CD44(high)CD62L(high)) CD8(+) T cells were identified in B6 mice with ongoing GVHD, with effector memory CD8(+) T cells as the dominant (>80%) population. Administration of these allogeneic memory CD8(+) T cells into secondary B6 recipients caused virulent GVHD. A similar allogeneic memory CD4(+) T cell population with the ability to mediate persistent GVHD was also identified in BALB/b mice receiving minor histocompatibility Ag-mismatched B6 T cell-replete bone marrow transplantation. These results indicate that allogeneic memory T cells are generated in vivo during GVH reactions and are able to cause GVHD, resulting in persistent host tissue injury. Thus, in vivo blockade of both alloreactive effector and memory T cell-mediated host tissue injury may prove to be valuable for GVHD prevention and treatment.     

CD11b expression identifies CD8+CD28+ T lymphocytes with phenotype and function of both naive/memory and effector cells

A previously unreported CD8(+)CD28(+)CD11b(+) T cell subset occurs in healthy individuals and expands in patients suffering from primary viral infections. In functional terms, these cells share the features of naive/memory CD8(+)CD28(+)CD11b(-) and terminally differentiated effector CD8(+)CD28(-)CD11b(+) subpopulations. Like CD28(-) cells, CD28(+)CD11b(+) lymphocytes have the ability to produce IFN-gamma, to express perforin granules in vivo, and to exert a potent cytolytic activity. Moreover, these cells can respond to chemotactic stimuli and can efficiently cross the endothelial barrier. In contrast, like their CD11b(-) counterpart, they still produce IL-2 and retain the ability to proliferate following mitogenic stimuli. The same CD28(+)CD11b(+) subpopulation detected in vivo could be generated by culturing naive CD28(+)CD11b(-) cells in the presence of mitogenic stimuli following the acquisition of a CD45RO(+) memory phenotype. Considering both phenotypic and functional properties, we argue that this subset may therefore constitute an intermediate phenotype in the process of CD8(+) T cell differentiation and that the CD11b marker expression can distinguish between memory- and effector-type T cells in the human CD8(+)CD28(+) T cell subset.