Qualitative differences between naïve and memory T cells

Mature T cells are produced in the thymus and released into the bloodstream in low numbers. These cells are considered to be immunologically naïve until such time as they encounter MHC-peptide complexes for which their T-cell receptors (TCR) have high affinity. Recognition of antigen in appropriate form, i.e. in association with costimulatory signals on the surface of professional antigen-presenting cells (APCs), leads to extensive T-cell proliferation and differentiation into effector cells. Once the infection has been cleared, it is no longer of benefit to the host to maintain high numbers of effector cells and most of the activated T cells die by apoptosis. However, a proportion of these cells survive, leaving the frequency of cells specific for the priming antigen much higher among memory T cells than that which existed among naïve T cells. This difference in frequency makes a major contribution to the nature of the secondary response, which is typically faster and of greater magnitude than the primary response. In addition, T cells may also carry a true ‘memory’ of a prior response to antigen, exhibiting differences from naïve T cells at the single cell level. Here we provide a brief overview of the qualitative differences that have been reported to exist between naïve and memory T cells and evidence that memory T cells themselves are functionally heterogeneous. The supposition that naïve and memory T cells can be distinguished phenotypically is based on the notion that memory T cells retain a permanent imprint of having responded to antigen. Precise identification of memory T cells, however, remains problematic. Unlike B cells, T cells do not appear to mutate their antigen receptor genes during the course of an immune response. Furthermore, discrimination between effector and memory T cells is accomplished on the basis of rather nebulous criteria; memory T cells are considered to differ from effector T cells by their continued survival after the acute immune response has died down and by being in a lower state of activation. As discussed further below, these distinctions are becoming increasingly blurred. Despite these difficulties, a number of phenotypic differences between naïve and memory T cells have been noted. Most of these are changes that arise during initial T-cell activation and appear to persist in memory cells. Especially prominent are differences in the cell surface expression of adhesion molecules between naïve and memory T cells. Thus, compared to naïve T cells, memory T cells have been reported to express higher levels of β1 (CD29, CD49d and CD49e) and β2 (CD11a, CD11b and CD18) integrins, CD2, CD44, CD54 and CD58.1–11 Increased expression of adhesion molecules on recently activated T cells reflects the requirements for effector T cells to enter peripheral tissues at sites of inflammation and interact with target cells, and may similarly affect the function of some memory T cells (see below). The expression of other molecules involved in lymphocyte migration also differs between naïve and memory T cells. Of particular interest are differences in the expression of two key molecules required for the entry of T cells into lymph nodes through high endothelial venules (HEVs): CD62L and CCR7. CD62L binds to vascular addressins expressed on HEVs and is responsible for the initial stage of adherence of blood-borne T cells to HEVs,12 while the CCR7 chemokine receptor controls responsiveness to chemokines expressed in HEVs at sites of lymphocyte entry.13 Whereas naïve T cells are uniform in expressing high levels of both molecules, some memory cells lose expression of CD62L and/or CCR7.14–17 However, memory T cells may express receptors for chemokines that direct them to inflammatory sites and for molecules involved in homing to peripheral tissues, such as the cutaneous lymphocyte antigen (CLA) which is involved in lymphocyte migration to skin.17 Other cell surface molecules that have been reported to distinguish between naïve and memory T cells include the IL-2R β-chain (CD122),18,19 Ly-6C19–21 and the common leukocyte antigen (CD45).4,6,14,22–28 CD122 is a component of both the IL-2R and the IL-15R and may play a role in the maintenance of memory T cells (see below),29 while Ly-6C is a low-molecular-weight (MW) glycosylphophatidylinositol-anchored molecule that has been proposed to participate in intercellular adhesion;30 both CD122 and Ly-6C are expressed at high levels on CD8+ memory T cells in the mouse. For CD45, which is a tyrosine phosphatase that regulates signalling through antigen receptors and cytokine receptors,31,32 it is the form of the molecule that differs between naïve and memory cells, rather than the level of expression. Multiple isoforms of CD45 are generated by differential splicing of three extracellular exons (A, B and C). These restricted (R) isoforms can be detected specifically with mAb directed against the variably spliced exons. Naïve T cells express the highest MW isoform, containing all three of these exons (commonly referred to as CD45RA in humans). During the course of T-cell activation, T cells switch to expressing lower-MW isoforms; in humans at least, activated T cells express the isoform of CD45 lacking all three variably spliced exons (defined as CD45R0). In many different species, expression of low-MW isoforms of CD45 is retained on memory cells. Since many of the phenotypic properties associated with memory T cells are in fact acquired soon after activation, these markers cannot be used on their own to discriminate between recently activated cells and memory cells. This distinction can be aided to a certain extent by combining these phenotypic markers with other criteria to exclude T cells that are actively responding to antigen. For example, it is generally assumed that memory T cells do not have a blasted morphology and do not express transient markers of activation such as CD69. Furthermore, differences in cell surface glycosylation have been reported to exist between effector and memory CD8+ T cells in mice. Specifically, memory cells have a higher degree of sialylation on 1 O-glycans and express lower levels of 2 O-glycans than effector cells;33–36 this difference can be detected using an antibody that binds specifically to CD43 only when this molecule has been modified by 2 O-glycans.37 However, in using signs of recent activation as exclusion criteria for memory T cells, it must be borne in mind that even long-term memory cells appear to be more metabolically active than naïve T cells.38 As discussed below, memory T cells undergo periodic rounds of cell division even in the complete absence of antigen. Therefore, markers of recent activation cannot be used definitively to distinguish between effector and memory T cells. Although memory T cells are enriched amongst cells expressing the surface markers discussed above, it is also clear that memory cells exhibit substantial phenotypic heterogeneity. This issue has received considerable attention in recent years, with interest stemming largely from a report that CD45R0+ T cells in human blood can be divided into CD62L+CCR7+ and CD62L−CCR7− subpopulations.17 These cells have been termed ‘central memory’ and ‘effector memory’ cells, respectively, based on their expression of lymph node homing molecules and their functional properties (see below). In addition to CD62L and CCR7, memory T cells may express other markers associated with naïve T cells, such as high-MW isoforms of CD45.16,17,39–46 In some instances, it is evident that expression of a ‘naïve’ phenotype by primed cells represents phenotypic reversion. This has been shown to be the case for CD62L, CCR7 and high-MW isoforms of CD45, each of which can be re-expressed by cells that were formerly negative for these markers.39,41,47–52 Phenotypic reversion occurs at different rates in different species and also differs for CD4 vs. CD8 cells. For example, rat CD45RC− (memory-phenotype) CD4+ T cells re-express CD45RC within 1 week when transferred to secondary recipients in the absence of antigen;39,41 the rapidity with which this reversion takes place suggests that the CD45RC– phenotype in the rat is a marker of recent activation rather than memory. Conversely, CD4+ memory T cells in mice can maintain a CD45RBlow phenotype for at least 10 weeks in the absence of antigen, although CD8+ T cells re-express CD45RB soon after activation.53 Phenotypic reversion is presumed to reflect a ‘cooling down’ of activated cells; lack of contact with antigen results in a return to a resting state and the loss of expression of activation molecules. By corollary, retention of memory markers may be indicative of periodic contact with persisting antigen. However, some phenotypic markers, particularly the expression of high levels of CD44 on mouse memory T cells, appear to be retained long-term in the complete absence of antigen.54,55 In addition, mechanisms other than reversion may account for some of the phenotypic heterogeneity observed amongst memory T cells. In this respect, it is notable that some CD45RA+ CD8+ T cells in human peripheral blood exhibit the properties of activated effector cells.11 These cells, which also express low levels of CD28 and CD27, appear to arise from chronic antigenic stimulation.56,57 Likewise, some primed CD45RA+ CD4+ T cells can be found under conditions of chronic antigen exposure.45 Whether the CD45RA+ cells observed in these studies have in fact re-expressed this molecule is unclear. Another possibility is that some cells may retain expression of CD45RA under certain conditions of activation. This is worth considering in view of data showing that mouse CD8+ T cells can differentiate directly into cells with the properties of memory cells antigenic in In these brief of CD8+ T cells to antigen by in low of generated T cells that retained expression of CD62L and CCR7 and which effector By cells to high of after antigenic expression of CD62L and CCR7 and into effector cells. Therefore, of all of the markers typically associated with T-cell activation is not an of antigenic the of phenotypic markers to distinguish between naïve and memory T cells is the fact that naïve T cells can markers of memory cells in the absence of antigenic This has been to when numbers of naïve cells are transferred into these naïve T cells expression of markers and exhibit effector this response is not by specific antigen by complexes in with The contribution of cells generated by this to the of T cells in mice is unclear. However, the fact that T cells are observed in mice that most T cells are from antigenic it remains that the phenotypic associated with proliferation may a contribution to the of T cells under conditions of has been evident for many that T cells to antigen in a different from T In recent years, T cells have evidence that this is also true for naïve and memory cells. are at two memory T cells appear to have requirements for activation than naïve T cells. This may include an to to lower of antigen than naïve T although some have to difference between naïve and memory T cells in their to In addition, memory T cells are on costimulatory signals than naïve T cells, and do not as a of antigenic of these may to the fact that a of cells can as for memory T cells compared to naïve T cells. Thus, while activation of naïve T cells is on antigen by cells memory T cells to antigen on other resting B they have been the response of naïve and memory T cells also For example, is some evidence that memory T cells faster and higher numbers in than naïve T cells antigenic However, this issue remains as these differences have not been detected in other Conversely, it is a that memory T cells effector after activation than naïve T cells. This the expression of CD8+ T and the of other than as to the basis of the responsiveness of memory T cells have been For the expression of effector by memory T cells may be to the of of for cytokine this has been shown to initial T-cell activation and is by some memory T cells have been shown to express for effector prior to secondary In addition, memory T cells a higher of both and than naïve T cells, that memory cells may be resting in the rather than the of the cell This for faster entry into and of by memory T cells Furthermore, differences between naïve and memory T cells have been that affect signalling within these cells. Specifically, naïve and memory T cells have been shown to differ with to and association of signalling of the expression of the molecule and association of CD45 with the Another that to the properties of memory T cells is an to interact with compared to naïve T cells. their expression of adhesion molecules (see it that memory cells form higher with other cells; this to the of by memory vs. naïve T cells. Furthermore, is evidence that T cells are on the basis of expressing with higher for antigen during the course of an immune This is a of the loss of cells expressing with the rates for Since genes do not undergo this is not directly to the involved in the of memory B cells, and results in in affinity. the data that memory T cells may a cell having a higher for antigen than the of naïve cells. As discussed above, memory T cells are phenotypically heterogeneous. the extent to which this heterogeneity also to the functional properties of memory cells remains to be it is evident that memory cells and effector memory cells exhibit clear differences in their response to antigen. Thus, a direct of these two that CD4+ CD45R0+ effector memory cells a of and within of through the while CD4+ CD45R0+ memory cells only In addition, effector memory cells were also to in response to lower of than memory cells, although the were more than naïve T cells. Therefore, memory T cells are in fact functionally and distinction between the functional properties of naïve and memory T cells is an This heterogeneity may account for in the differences between naïve and memory cells. migration is by the of adhesion molecules and chemokine receptors expressed by Since the expression of these molecules differs between naïve and memory T cells, it is not that these cells have different Naïve T cells exhibit a restricted of in which they between the secondary lymph nodes and blood and through takes The expression of CD62L and CCR7 by naïve T cells a key role in this Thus, entry of naïve T cells into lymph nodes and occurs at HEVs in a and (see In addition, although initial entry of into the is a with T cells being from the blood into the migration of cells into the T-cell of the may also expression of However, naïve T cells lack homing receptors for peripheral tissues and chemokine receptors for inflammatory and are to enter By some memory T cells express chemokine and adhesion receptors that them to into into peripheral tissues for T cells in and for memory T cells in However, as discussed above, memory T cells are with to the expression of homing molecules and exhibit heterogeneity in memory cells, which express CD62L and CCR7, are to enter lymph nodes HEVs and exhibit a of migration that is to that of naïve T cells. Conversely, effector memory cells lack CD62L and CCR7 express homing receptors that them to enter these cells may lymph nodes lymph rather than through In addition, of memory T cells exhibit a for migration to particular tissues, for vs. on the basis of the particular homing molecules This for specific tissues may be by the in which initial T-cell priming The migration by naïve and memory T cells are to their activation requirements and functional As above, activation of naïve T cells antigen on These are found in the T-cell of secondary into these and antigen the of in peripheral secondary as for naïve T cells and that the of T cells their specific antigen. of naïve T cells between different these cells to the surface of from all of the In addition, the of the tissues is for of activated T cells and their differentiation into Conversely, activation of memory T cells can the conditions of the secondary both to the frequency of cells among memory cells and to their to activated by a of As a effector memory T cells can a function at sites of and provide a response to In memory T cells, which appear to in a to naïve T cells, to antigen in secondary Whether memory cells antigen on in to undergo activation and differentiation into effector cells is (see it that a much greater degree of antigenic in Therefore, and effector memory T cells may provide secondary with effector memory cells an response and memory cells numbers of In cell can be at numbers by two survival of cells that do not and proliferation of cells, which is by the of cell the of T-cell is that these cells can in for of This shown in T-cell proliferation in by of a this of the time for T cells in the of mice to be of the of recent studies using as a have shown that human T cells also the for CD4+ and CD8+ T cells in peripheral blood were to be and in a number of different species, and have shown that T cells at a faster than T cells. the of of is faster for cells than cells. of these data directly to naïve and memory T cells is by the of using phenotypic markers to these cells (see However, it is notable that studies of cell that can be as memory T cells with high cells at after using to cells have shown that these cells exhibit rates of division in to T cells in The is that memory T cells are by cell division while naïve T cells persist as cells. Although T cells exhibit some memory T cells appear to in a state for of This is evident from the results of which considerable heterogeneity in the rates at which cells during the lose after the of This is not in view of the evidence for heterogeneity amongst memory T cells. it is and memory T cells to of cells that can be distinguished phenotypically vs. effector memory T In addition to exhibiting naïve and memory T cells are also on different for their have been in this and The issue of long-term survival of naïve T cells is on contact with has been expression in the thymus is for the initial of T cells, the role of peripheral in T-cell survival of T cells into and transient expression of in the thymus of such mice. In these studies have shown that the of naïve CD4+ and CD8+ T cells is in the absence of However, it is notable that naïve CD4+ T cells can for considerable of time in the absence of Thus, the of naïve CD4+ cells in the absence of has been to be weeks by some while have reported that naïve CD4 cells can in and for the weeks after These results that naïve CD4 cells only contact with in to The nature of the involved in survival signals to naïve T cells is For proliferation (see T cells to in a with the involved being at least to involved in T-cell in the Therefore, possibility is that the MHC-peptide that proliferation in survival under Conversely, survival of naïve T cells may only of and be of the has been extensive the role of persisting antigen in the maintenance of T-cell memory. it remains a issue periodic contact with antigen is required to maintain it is generally that the survival of memory T cells is Thus, studies in mice have shown that both CD4+ and CD8+ memory T cells of into Furthermore, long-term survival also to CD4+ and CD8+ memory T cells transferred to and Therefore, naïve T cells, memory T cells do not appear to on with for memory T cells also to in For CD8+ memory T cells, proliferation occurs at rates after to that survival is by signals of By it is clear the functional properties of memory T cells are also of contact with For some properties of memory T cells, this appear to be the Thus, CD4+ and CD8+ memory T cells retain a phenotype and the for of after to However, it has been reported that CD4 memory cells in lose other functional to memory cells, the to to antigen by such as B cells and a from Whether a for contact with for CD8 memory cells remains to be The cytokine that has been in the survival of naïve T cells is In to be for naïve T cell as these cells to recipients in mice with in addition, naïve T cells to in Although other such as and have been shown to naïve T cells from in and these do not appear to be required for the survival of naïve T cells in and cannot for the absence of In addition to the survival of naïve T cells in also to be required for proliferation of naïve T cells under Thus, maintenance of naïve T cells to be on contact with both and that play a role in the maintenance of memory T cells from studies showing that of of proliferation of CD8+ T cells in a number of other and were found to have proliferation restricted to CD8+ T cells of each of these no cell division amongst naïve phenotype T cells. Of the shown to proliferation of CD8+ T cells in however, only to these cells to when to T cells in the proliferation of CD8+ T cells in response to associated with much higher expression of the IL-15R β-chain on vs. CD8+ T Since and are all of expression by two through of it has been proposed that as a common effector molecule the on CD8+ T cell on the that of of expression in results in proliferation of CD8+ T cells, it that the high proliferation of these cells in mice be to of In of this of into mice shown to the proliferation of CD8+ T Although this antibody can signalling through both the IL-15R and the the to from of CD8+ T-cell The from this that and on the proliferation of CD8 T cells in with and cell the number of CD8+ T cells is in both and and in mice Thus, to play an role in the number of CD8+ T cells found in evidence of for CD8 memory T cells further in controls the maintenance of CD8+ T cells remains unclear. The is that proliferation is to a of in the of the more of CD8+ T cells, However, the fact that a considerable proportion of CD8+ T cells can persist for of time in a state is with the that all memory cells are Therefore, an possibility is that is to survival as as proliferation of CD8+ T cells. In this respect, it is notable that expression of the in CD8+ T In to on CD8+ T cells, proliferation of CD4+ T cells in in Furthermore, CD4+ T cells are found in numbers in and Therefore, not appear to play a role in the maintenance of CD4+ memory cells. In CD4+ memory cells are in mice for the common that of the this receptor component and are for the survival of CD4+ memory T Thus, do play a role in the maintenance of CD4+ memory T cells, the involved are from that have on naïve T cells CD8+ memory cells. the evidence that the of CD4+ memory T cells can be by from a in which T-cell proliferation the activation of T cells in these the proliferation of CD4+ T cells through a that on is considerable evidence to the that T cells retain a permanent imprint of a prior response to antigen. In naïve and memory T cells differ in of responsiveness to antigen and and requirements for However, it is also clear that memory T cells are with to all of these The recent identification of markers that can be used to of memory T cells a more of the of memory cells. amongst memory T cells may arise as a of the initial conditions of activation, also be by the in which memory T cells In this respect, at the between phenotypically of memory T cells provide into the nature of T-cell memory. For example, while it is evident that CD4+ memory cells can differentiate into cells effector memory cells with antigen in the at which this takes place in and reversion from effector to memory cell in the absence of such are not In addition, the identification of memory T cells expressing certain markers that are of naïve T cells the of some memory T cells may in fact be from naïve T cells in it be of interest to these cells retain functional of memory cells. the extent to which the functional properties of memory T cells to the of the secondary immune response remains unclear. In changes at the single cell level with in frequency in a response to secondary infection effector memory T cells are to to on the basis of their activation requirements and and the of the memory T cells are of a number of effector cells based on their high an to differentiate into may also play a These effector cells and to clear the memory T cells are not only at a high frequency also in a state of functional is for both of the immune and for the of of qualitative and changes in memory T cells in a secondary response to memory T cells are in peripheral tissues at sites of These cells exhibit effector function of antigen on and the of memory T cells are activated in secondary the lymph of antigen on and numbers of effector cells. The response of memory cells is as a of both a high frequency of antigen specific cells and their to differentiate into This is number from the for