Activated CD8 T cells redistribute to antigen-free lymph nodes and exhibit effector and memory characteristics

Exogenous dendritic cells display restricted trafficking when injected in vivo and stimulate CD8 T cell responses that are localized to a small number of lymphoid compartments. By examining these responses in the presence and absence of FTY720, a drug that causes sequestration of T cells in lymph nodes, we demonstrate that a significant fraction of divided CD8 T cells redistribute into Ag-free lymph nodes within 3 days of activation. Despite variation in the level of expression of CD62L, redistribution of these cells is CD62L-dependent. Redistributed CD8 T cells exhibit characteristics of differentiated effectors. However, when re-isolated from Ag-free lymph nodes 3 days after activation and transferred into naive mice, they persist for at least 3 wk and expand upon Ag challenge. Thus, CD8 T cells that redistribute to Ag-free lymph nodes 3 days after immunization contain memory precursors. We suggest that this redistribution process represents an important mechanism for establishment of lymph node resident central memory, and that redistribution to Ag-free nodes is an additional characteristic to be added to those that distinguish memory precursors from terminal effectors.     

CD8 T cell recall responses are regulated by the tissue tropism of the memory cell and pathogen

Whether memory CD8 T cells can be reactivated in nonlymphoid tissues is unclear. Using mice lacking the spleen, lymph nodes, or both, we show that the secondary T cell response, but not homeostatic maintenance of memory cells, required lymphoid tissue. Whereas primary and secondary CD8 T cell responses to vesicular stomatitis virus infection were lymph node dependent, responses to Listeria monocytogenes infection were driven primarily in the spleen. Memory cell subset reactivation was also regulated by location of the responding population and the pathogen. Thus, CD62Llow effector memory T cells (TEM) cells responded nearly as well as CD62Lhigh central memory T cells (TCM) and TCM cells after L. monocytogenes infection, and both subsets generated equivalent populations of secondary memory cells. In contrast, TCM cells, but not TEM cells, mounted a robust response to vesicular stomatitis virus infection. TCM and TEM cells also required lymphoid tissue to mount recall responses, and the bone marrow did not contribute significantly to the response of either subset. Our findings indicated that characteristics of the infectious agent and the migratory preferences of memory cells dictated the secondary lymphoid tissue requirement for the recall response to infection.     

Relationship of germinal centers in lymphoid tissue to immunologic memory. VI. Transfer of B cell memory with lymph node cells fractionated according to their receptors for peanut agglutinin

We isolated germinal center B cells by exploiting their high affinity for peanut agglutinin (PNA). The PNA+ and PNA- B cells, fractionated by panning on PNA-coated petri dishes, were examined for their ability to transfer memory responses to irradiated recipients at various times after priming. With such fractionated B cells from lymph nodes taken at the peak of germinal center formation, the largest response was obtained in recipients of the PNA+ B cell population. At 4 to 5 wk after priming, and 10 days after challenge with an unrelated antigen, memory responses were approximately equal in recipients of PNA+ or PNA- B cells. At 14 wk after priming, memory responses were found only in recipients of the PNA- B cell population. Memory B cells from the spleen, taken from mice primed in the footpad 8 wk earlier, were also PNA-. Finally, we show that boosting with a TNP-conjugate in the footpad, 6 mo after priming in the same footpad, induced the reappearance of marked memory responsiveness in the PNA+ B cell fraction of the draining node.     

The Initial Draining Lymph Node Primes the Bulk of the CD8 T Cell Response and Influences Memory T Cell Trafficking after a Systemic Viral Infection

Lymphocytic choriomeningitis virus (LCMV) causes a systemic infection in mice with virus replication occurring in both peripheral tissues and secondary lymphoid organs. Because of the rapid systemic dissemination of the virus, the secondary lymphoid organs responsible for the induction of the LCMV-specific CD8 T cell response are poorly defined. We show that the mediastinal lymph node (MedLN) serves as the primary draining lymph node following LCMV infection. In addition, we demonstrate that the MedLN is responsible for priming the majority of the virus-specific CD8 T cell response. Following resolution of the acute infection, the draining MedLN exhibits characteristics of a reactive lymph node including an increased presence of germinal center B cells and increased cellularity for up to 60 days post-infection. Furthermore, the reactive MedLN harbors an increased frequency of CD62L⁻ effector memory CD8 T cells as compared to the non-draining lymph nodes. The accumulation of LCMV-specific CD62L⁻ memory CD8 T cells in the MedLN is independent of residual antigen and is not a unique feature of the MedLN as footpad infection with LCMV leads to a similar increase of virus-specific CD62L⁻ effector memory CD8 T cells in the draining popliteal lymph node. Our results indicate that CD62L⁻ effector memory CD8 T cells are granted preferential access into the draining lymph nodes for an extended time following resolution of an infection.     

Tolerance induction of IgG+ memory B cells by T cell-independent type II antigens

Memory B cells generated during a T cell-dependent immune response rapidly respond to a secondary immunization by producing abundant IgG Abs that bind cognate Ag with high affinity. It is currently unclear whether this heightened recall response by memory B cells is due to augmented IgG-BCR signaling, which has only been demonstrated in the context of naive transgenic B cells. To address this question, we examined whether memory B cells can respond in vivo to Ags that stimulate only through BCR, namely T cell-independent type II (TI-II) Ags. In this study, we show that the TI-II Ag (4-hydroxy-3-nitrophenyl) acetyl (NP)-Ficoll cannot elicit the recall response in mice first immunized with the T cell-dependent Ag NP-chicken γ-globulin. Moreover, the NP-Ficoll challenge in vivo as well as in vitro significantly inhibits a subsequent recall response to NP-chicken γ-globulin in a B cell-intrinsic manner. This NP-Ficoll-mediated tolerance is caused by the preferential elimination of IgG(+) memory B cells binding to NP with high affinity. These data indicate that BCR cross-linking with a TI-II Ag does not activate IgG(+) memory B cells, but rather tolerizes them, identifying a terminal checkpoint of memory B cell differentiation that may prevent autoimmunity.     

Functional responses and costimulator dependence of memory CD4+ T cells

To examine the functional characteristics of memory CD4+ T cells, we used an adoptive transfer system to generate a stable population of Ag-specific memory cells in vivo and compared their responses to Ag with those of a similar population of Ag-specific naive cells. Memory cells localized to the spleen and lymph nodes of mice and exhibited extremely rapid recall responses to Ag in vivo, leaving the spleen within 3-5 days of Ag encounter. Unlike their naive counterparts, memory cells produced effector cytokines (IFN-gamma, IL-4, IL-5) within 12-24 h of Ag exposure and did not require multiple cycles of cell division to do so. Memory cells proliferated at lower Ag concentrations than did naive cells, were less dependent on costimulation by B7 molecules, and independent of costimulation by CD40. Furthermore, effector cytokine production by memory cells also occurred in the absence of either B7 or CD40 costimulation. Lastly, memory cells were resistant to tolerance induction. Together, these findings suggest that the threshold for activation of memory CD4+ cells is lower than that of naive cells. This would permit memory cells to rapidly express their effector functions in vivo earlier in the course of a secondary immune response, when the levels of Ag and the availability of costimulation may be relatively low.     

Transfer of memory cells into antigen-pretreated hosts. I. Functional detection of migration sites for antigen-specific B cells.

The distribution of functionally active hapten-specific B memory cells was investigated. Using antigen-pretreated lethally irradiated recipients, a marked accumulation of adoptively transferred B memory cells was demonstrated in lymph nodes containing specific antigen, but not in lymph nodes containing non-cross-reacting hapten conjugates. This difference in responsiveness between lymph nodes containing specific versus those containing nonspecific antigen developed over a period 3–5 days after memory cell transfer. The localization of antigen specific cells was T-cell independent; both carrier-primed T helper cells and specific antigenic challenge, however, were required to trigger the localized B memory cells into antibody production. Specific B memory cell accumulation did not result from an expansion of the antigen-specific cell population due to local proliferation induced by antigen depots in the lymph nodes to challenge. Rather, the results indicated that recirculating B memory cells had progressively accumulated through retention by antigen in the lymph node. These findings suggest that, in the absence of T-cell help and specific antigenic challenge, B memory cells accumulate in lymphoid tissue (follicles) without responding and provide persistent local memory for the humoral immune response.     

Spread of herpes simplex virus in lymph nodes after experimental infection of mice

Herpes simplex virus was frequently isolated from ipsilateral popliteal lymph nodes after percutaneous inoculation of the dorsal face of the footpad, and from ipsi- and contralateral submandibular lymph nodes after percutaneous inoculation of the cheek or the orofacial area of mice. Virus was detected only on very rare occasion in nondraining lymph nodes (inguinal or axillary) or in contralateral popliteal lymph nodes, but was frequently isolated in contralateral lumbar lymph nodes after footpad inoculation. The presence of virus in lymph nodes paralleled or followed the invasion of ipsilateral sensory ganglia and was associated with dissemination of virus in contralateral sensory ganglia after unilateral inoculation. In older mice virus was detected only occasionally in lymph nodes and dissemination of virus in contralateral sensory ganglia was generally not observed. The results suggest that lymphatic spread may contribute to dissemination of virus in contralateral sensory ganglia after unilateral inoculation of mice.     

Activation, differentiation, and migration of naive virus-specific CD8+ T cells during pulmonary influenza virus infection

The low precursor frequency of individual virus-specific CD8(+) T cells in a naive host makes the early events of CD8(+) T cell activation, proliferation, and differentiation in response to viral infection a challenge to identify. We have therefore examined the response of naive CD8(+) T cells to pulmonary influenza virus infection with a murine adoptive transfer model using hemagglutinin-specific TCR transgenic CD8(+) T cells. Initial activation of CD8(+) T cells occurs during the first 3 days postinfection exclusively within the draining lymph nodes. Acquisition of CTL effector functions, including effector cytokine and granule-associated protease expression, occurs in the draining lymph nodes and differentially correlates with cell division. Division of activated CD8(+) T cells within the draining lymph nodes occurs in an asynchronous manner between days 3 and 4 postinfection. Despite the presence of Ag for several days within the draining lymph nodes, dividing T cells do not appear to maintain contact with residual Ag. After multiple cell divisions, CD8(+) T cells exit the draining lymph nodes and migrate to the infected lung. Activated CD8(+) T cells also disseminate throughout lymphoid tissue including the spleen and distal lymph nodes following their emigration from draining lymph nodes. These results demonstrate an important role for draining lymph nodes in orchestrating T cell responses during a local infection of a discrete organ to generate effector CD8(+) T cells capable of responding to infection and seeding peripheral lymphoid tissues.     

Germinal center B cells and antibody production in the bone marrow

In secondary antibody (Ab) responses, Ag processing and presentation occur in secondary lymphoid organs but most serum Ab is produced by cells in the bone marrow. Plasma cells in the bone marrow are derived from B cells activated by Ag in secondary lymphoid organs. We hypothesized that germinal center (GC) B cells, which acquire Ag from follicular dendritic cells in draining lymph nodes during the first few days of the secondary response, migrate to the bone marrow to terminally differentiate and produce specific Ab. To test this we looked for GC B cells in the thoracic duct lymph and in peripheral blood after secondary challenge using the peanut agglutininhi phenotype and blast cell morphology as markers for GC B cells. In addition, GC B cells were injected i.v. into naive recipients to determine if they would home to the bone marrow. Finally, to determine if the bone marrow environment supports maturation and Ab production by GC B cells, we cocultured GC B cells with bone marrow cells or bone marrow supernatants. The results indicate that blast cells bearing the GC B cell phenotype were present in both the thoracic duct and the peripheral blood 3 days after antigenic challenge. Day 3 peripheral blood cells secreted specific Ab, whereas cells isolated on day 0, 8, or 11 did not. Furthermore, in adoptive transfer experiments, only the day 3 GC B cells produced specific Ab and migrated to the bone marrow of naive mice. Finally, either bone marrow cells or factor(s) produced by bone marrow cells markedly enhanced Ab production by day 3 GC B cells. These data support the hypothesis that during the first few days after secondary challenge GC B cells seed the bone marrow and differentiate into plasma cells which produce the large quantities of Ab typical of secondary responses.     

Control of memory CD4 T cell recall by the CD28/B7 costimulatory pathway

The CD28/B7 costimulatory pathway is generally considered dispensable for memory T cell responses, largely based on in vitro studies demonstrating memory T cell activation in the absence of CD28 engagement by B7 ligands. However, the susceptibility of memory CD4 T cells, including central (CD62L(high)) and effector memory (T(EM); CD62L(low)) subsets, to inhibition of CD28-derived costimulation has not been closely examined. In this study, we demonstrate that inhibition of CD28/B7 costimulation with the B7-binding fusion molecule CTLA4Ig has profound and specific effects on secondary responses mediated by memory CD4 T cells generated by priming with Ag or infection with influenza virus. In vitro, CTLA4Ig substantially inhibits IL-2, but not IFN-gamma production from heterogeneous memory CD4 T cells specific for influenza hemagglutinin or OVA in response to peptide challenge. Moreover, IL-2 production from polyclonal influenza-specific memory CD4 T cells in response to virus challenge was completely abrogated by CTLA4Ig with IFN-gamma production partially inhibited. When administered in vivo, CTLA4Ig significantly blocks Ag-driven memory CD4 T cell proliferation and expansion, without affecting early recall and activation. Importantly, CTLA4Ig treatment in vivo induced a striking shift in the phenotype of the responding population from predominantly T(EM) in control-treated mice to predominantly central memory T cells in CTLA4Ig-treated mice, suggesting biased effects of CTLA4Ig on T(EM) responses. Our results identify a novel role for CD28/B7 as a regulator of memory T cell responses, and have important clinical implications for using CTLA4Ig to abrogate the pathologic consequences of T(EM) cells in autoimmunity and chronic disease.     

Fc gamma receptor IIB on follicular dendritic cells regulates the B cell recall response

Generation of the B cell recall response appears to involve interaction of Ag, in the form of an immune complex (IC) trapped on follicular dendritic cells (FDCs), with germinal center (GC) B cells. Thus, the expression of receptors on FDC and B cells that interact with ICs could be critical to the induction of an optimal recall response. FDCs in GCs, but not in primary follicles, express high levels of the IgG Fc receptor Fc gamma RIIB. This regulated expression of Fc gamma RIIB on FDC and its relation to recall Ab responses were examined both in vitro and in vivo. Trapping of IC in spleen and lymph nodes of Fc gamma RII-/- mice was significantly reduced compared with that in wild-type controls. Addition of ICs to cultures of Ag-specific T and B cells elicited pronounced Ab responses only in the presence of FDCs. However, FDCs derived from Fc gamma RIIB-/- mice supported only low level Ab production in this situation. Similarly, when Fc gamma RIIB-/- mice were transplanted with wild-type Ag-specific T and B cells and challenged with specific Ag, the recall responses were significantly depressed compared with those of controls with wild-type FDC. These results substantiate the hypothesis that FcgammaRIIB expression on FDCs in GCs is important for FDCs to retain ICs and to mediate the conversion of ICs to a highly immunogenic form and for the generation of strong recall responses.     

CD4+ and CD8+ T cells exhibit differential requirements for CCR7-mediated antigen transport during influenza infection

Upon encounter of viral Ags in an inflammatory environment, dendritic cells up-regulate costimulatory molecules and the chemokine receptor CCR7, with the latter being pivotal for their migration to the lymph node. By utilizing mice deficient in CCR7, we have examined the requirement of dendritic cell-mediated Ag transport from the lung to the draining lymph node for the induction of anti-influenza immune responses in vivo. We found that CCR7-mediated migration of dendritic cells was more crucial for CD8(+) T cell than CD4(+) T cell responses. While no specific CD8(+) T cell response could be detected in the airways or lymphoid tissues during the primary infection, prolonged infection in CCR7-deficient mice did result in a sustained inflammatory chemokine profile, which led to nonspecific CD8(+) T cell recruitment to the airways. The recruitment of influenza-specific CD4(+) T cells to the airways was also below levels of detection in the absence of CCR7 signaling, although a small influenza-specific CD4(+) T cell population was detectable in the draining lymph node, which was sufficient for the generation of class-switched anti-influenza Abs and a normal CD4(+) T cell memory population. Overall, our data show that CCR7-mediated active Ag transport is differentially required for CD4(+) and CD8(+) T cell expansion during influenza infection.     

Antigen modulation of the immune response. V. Generation of large memory cells in antigen draining lymph nodes.

We have studied the distribution of memory B cell subpopulations by using 1g velocity sedimentation and adoptive transfer. When the non-antigen-draining mesenteric lymph nodes were examined 4 weeks after intraperitoneal immunization with DNPBGG, large memory cells were present in only very low numbers. However, when the draining parathymic nodes were removed, a significant enrichment of large memory cell activity was seen. When these results were corrected for the cell yields in each 1g separated fraction we found that 59% of the total memory cells were small, 36% medium and 5% large in the mesenteric lymph node preparations and 40% were small, 46% medium and 14% large in the parathymic lymph node suspensions. When popliteal lymph nodes were removed after footpad immunization, 32% of the total memory cell activity was in the small cell fraction while 49% was in the medium fraction and 18% in the large cell fraction. Control experiments were also run to show that the shift in the velocity sedimentation profile of the various memory cell populations was not an artifact of the adoptive transfer system nor a result of selective antigen triggering.From these results it would appear that the size distribution of memory cells depends upon the source of cells studied, large memory cells being found predominantly only in lymph nodes draining the site of antigen injection. Since the large memory cells can also be found in the thoracic duct lymph after footpad immunization but not after intraperitoneal immunization, it is suggested that the larger cells can circulate to other lymphoid tissues but cannot recirculate.     

CCR2 and CCR6, but not endothelial selectins, mediate the accumulation of immature dendritic cells within the lungs of mice in response to particulate antigen

Dendritic cells (DC) migrate from sites of inflammation to lymph nodes to initiate primary immune responses, but the molecular mechanisms by which DC are replenished in the lungs during ongoing pulmonary inflammation are unknown. To address this question, we analyzed the secondary pulmonary immune response of Ag-primed mice to intratracheal challenge with the particulate T cell-dependent Ag sheep erythrocytes (SRBC). We studied wild-type C57BL/6 mice and syngeneic gene-targeted mice lacking either both endothelial selectins (CD62E and CD62P), or the chemokine receptors CCR2 or CCR6. DC, defined as non-autofluorescent, MHC class II(+)CD11c(mod) cells, were detected in blood, enzyme-digested minced lung, and bronchoalveolar lavage fluid using flow cytometry and immunohistology. Compared with control mice, Ag challenge increased the frequency and absolute numbers of DC, peaking at day 1 in peripheral blood (6.5-fold increase in frequency), day 3 in lung mince (20-fold increase in total DC), and day 4 in bronchoalveolar lavage fluid (55-fold increase in total DC). Most lung DC expressed CD11c, CD11b, and low levels of MHC class II, CD40, CD80, and CD86, consistent with an immature myeloid phenotype. DC accumulation depended in part upon CCR2 and CCR6, but not endothelial selectins. Thus, during lung inflammation, immature myeloid DC from the bloodstream replace emigrating immature DC and transiently increase total intrapulmonary APC numbers. Early DC recruitment depends in part on CCR2 to traverse vascular endothelium, plus CCR6 to traverse alveolar epithelium. The recruitment of circulating immature DC represents a potential therapeutic step at which to modulate immunological lung diseases.     

Protective efficacy of a cloned Brugia malayi antigen in a mouse model of microfilaremia

Immunization of mice with irradiated Brugia larvae or parasite extracts has been shown to induce partial resistance to microfilaremia and enhance clearance of infective larvae. We recently reported the cloning of a 548 amino acid 62-kDa Brugia malayi Ag identified on the basis of reactivity with antisera to a subset of protective microfilarial Ag. Our study describes the protective efficacy against microfilaremia in mice, immunogenicity, and parasite stage-specificity of this candidate vaccine molecule. Immunization of Swiss or BALB/c mice with 1 to 3 micrograms of a 92-kDa trpE fusion protein encoding amino acids 1-479 reduced the intensity of microfilaremia by 40 to 60% compared to control animals given buffer or bacterial trpE (p less than 0.01 to 0.001). Mice immunized with the 92-kDa fusion protein developed delayed-type hypersensitivity reactivity to B. malayi as assessed by enhanced footpad swelling 24 and 48 h after intradermal injection of adult worm extract and in vitro lymph node mononuclear cell proliferation (3H-thymidine uptake) in response to the fusion protein (mean +/- SD stimulation index 4.7 +/- 0.8 vs 2.0 +/- 1.4 for trpE, p less than 0.05). Proliferative responses of lymph node cells coincubated with three other fusion proteins corresponding to the filarial protein truncated from its carboxyl-terminus suggest that dominant T cell epitopes of the 62-kDa Ag are encompassed by amino acids 437-479. Rabbit antibody to the 92-kDa trpE fusion protein immunoprecipitated a 62-kDa polypeptide from [35S] methionine biosynthetically labeled B. malayi microfilariae, adult female, and adult male worms. These data indicate that a recombinant Ag expressed in several developmental stages of B. malayi is capable of inducing partial resistance against microfilariae and Ag-specific T cell responses in mice.     

Phenotype and homing of CD4 tumor-specific T cells is modulated by tumor bulk

Technical difficulties in tracking endogenous CD4 T lymphocytes have limited the characterization of tumor-specific CD4 T cell responses. Using fluorescent MHC class II/peptide multimers, we defined the fate of endogenous Leishmania receptor for activated C kinase (LACK)-specific CD4 T cells in mice bearing LACK-expressing TS/A tumors. LACK-specific CD44(high)CD62L(low) CD4 T cells accumulated in the draining lymph nodes and had characteristics of effector cells, secreting IL-2 and IFN-gamma upon Ag restimulation. Increased frequencies of CD44(high)CD62L(low) LACK-experienced cells were also detected in the spleen, lung, liver, and tumor itself, but not in nondraining lymph nodes, where the cells maintained a naive phenotype. The absence of systemic redistribution of LACK-specific memory T cells correlated with the presence of tumor. Indeed, LACK-specific CD4 T cells with central memory features (IL-2(+)IFN-gamma(-)CD44(high)CD62L(high) cells) accumulated in all peripheral lymph nodes of mice immunized with LACK-pulsed dendritic cells and after tumor resection. Together, our data demonstrate that although tumor-specific CD4 effector T cells producing IFN-gamma are continuously generated in the presence of tumor, central memory CD4 T cells accumulate only after tumor resection. Thus, the continuous stimulation of tumor-specific CD4 T cells in tumor-bearing mice appears to hinder the systemic accumulation of central memory CD4 T lymphocytes.     

Pulmonary infection with influenza A virus induces site-specific germinal center and T follicular helper cell responses

Protection from influenza A virus (IAV) challenge requires switched, high affinity Abs derived from long-lived memory B cells and plasma cells. These B cell subsets are generated in germinal centers (GCs), hallmark structures of T helper cell-driven B cell immunity. A full understanding of the GC reaction after respiratory IAV infection is lacking, as is the characterization of T follicular helper (T(FH)) cells that support GCs. Here, GC B cell and T(FH) cell responses were studied in mice following pulmonary challenge with IAV. Marked GC reactions were induced in draining lymph nodes (dLNs), lung, spleen and nasal-associated lymphoid tissue (NALT), although the magnitude and kinetics of the response was site-specific. Examination of switching within GCs demonstrated IgG2(+) cells to compose the largest fraction in dLNs, lung and spleen. IgA(+) GC B cells were infrequent in these sites, but composed a significant subset of the switched GC population in NALT. Further experiments demonstrated splenectomized mice to withstand a lethal recall challenge, suggesting the spleen to be unnecessary for long-term protection in spite of strong GC responses in this organ. Final studies showed that T(FH) cell numbers were highest in dLNs and spleen, and peaked in all sites prior to the height of the GC reaction. T(FH) cells purified from dLNs generated IL-21 and IFNγ upon activation, although CD4(+)CXCR5(-) T effector cells produced higher levels of all cytokines. Collectively, these findings reveal respiratory IAV infection to induce strong T helper cell-driven B cell responses in various organs, with each site displaying unique attributes.     

Lack of ICAM-1 on APCs during T cell priming leads to poor generation of central memory cells

ICAM-1/LFA-1 interactions are known to enhance T cell/APC interactions and to promote T cell activation and cytokine secretion. We have analyzed the consequences of ICAM-1-mediated signaling on the generation of memory T cell subsets. We report that lack of ICAM-1 on APCs, but not on T cells, leads to poor T cell activation and proliferation in vitro and in vivo, and that the defect can be compensated by Ag dose, exogenous IL-2, additional costimulation, and by increasing responder T cell density on APCs. ICAM-1-null mice do not respond to immunization with OVA peptide, but immunization with OVA or with Salmonella typhimurium leads to good T cell proliferation 7-10 days later, and clearance of a challenge infection is equivalent to that of wild-type mice. However, when followed over time, recall proliferation and antibacterial immunity decay rapidly in ICAM-1-null mice, while recall cytokine responses are unaffected. The decline in immunity is not related to poor survival of T cells activated on ICAM-1-null APCs, or to poor generation of effectors in ICAM-1-null mice. Phenotypic analysis of T cells stimulated on ICAM-1-null APCs reveals preferential generation of CD44(high) CD62L(low) effector memory cells (T(EM)) over CD44(high) CD62L(high) central memory cells (T(CM)). Further, while the proportion of naive:memory T cells is similar in unmanipulated wild-type and ICAM-1-null mice, there is an accumulation of T(EM) cells, and a high T(EM):T(CM) ratio in aging ICAM-1-null mice. Together, the data indicate that signaling through LFA-1 during T cell activation may be involved in commitment to a proliferation-competent memory pool.