The response in old mice: positive and negative immune memory after priming in early age

Marta Sánchez, Karin Lindroth1,, Eva Sverremark2,, África González Fernández and Carmen Fernández1,

Área of Immunology, Faculty of Sciences, Vigo University, Vigo, 36.200 Pontevedra, Spain
1 Department of Immunology, The Wenner-Gren Institute, Stockholm University, 106 91 Stockholm, Sweden
2 Department of Medicine, Karolinska Hospital, 171 76 Stockholm, Sweden

Correspondence to: Correspondence to C. Fernández


    Abstract
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
To analyze the effect of age in both B and T cell compartments of the immune system, we have studied the anti-dextran (Dx) B512 humoral immune response in aged C57BL/6 mice. We have used Dx in its native form, which induces a thymus-independent (TI) response, or conjugated to chicken serum albumin (CSA), which induces a thymus-dependent (TD) response. We have also analyzed the adjuvant effect of cholera toxin (CT) in both types of responses. Our results show that the B cell compartment is not greatly affected by age as demonstrated in the TI responses and that CT is a powerful adjuvant despite the age of the animals. However, we found a severe age-associated impairment of TD responses. We conclude that the first antigenic challenge deeply influences further antigenic responses in a positive or negative manner. Priming in early life with native Dx (TI) inhibited late TD responses in aged mice, even when the primary immunization had occurred a long time ago. This negative memory affects posterior TD responses both in the quantity and in the affinity of the response. However, immunization at an early age with TD priming (CSA–Dx) provoked a long-lasting immune memory that abolished in part the age-associated impairment of the response. Our results suggest that protocols of vaccination with TI antigens may not be a convenient strategy, because the development of further optimal immune responses to the same antigen can be impaired.

Keywords: dextran, immune memory, immunosenescence, thymus dependent, thymus independent, vaccines


    Introduction
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
The immune response is often impaired in very young and very old people, and it is widely accepted that age is a crucial factor in the behavior of the immune system (1,2). Both the T (3,4) and the B cell (reviewed in 5) compartments seem to be affected. However, the impact of aging is more profound on T cell functions than with regard to B cell functions. This fact could partially be explained by the effects that thymic involution may have in the maintenance and renewal of functional T cell repertoires (6). In the elderly, not only primary and secondary antibody responses are reduced in magnitude (710), but it has also been described that the antibodies generated after antigenic challenge are less protective compared to the antibodies produced by young adults (11). Changes in the Ig repertoire leading to the generation of antibodies of lower affinity, and the appearance and accumulation of autoreactive antibodies have been proposed to be some of the contributing factors to the senescence of the immune system (12,13). This could explain why aged individuals are more susceptible to infections, and also more prone to develop lymphoproliferative and autoimmune disorders (1).

The aim of this study was to investigate the effect of aging on the immune response to thymus-independent (TI) and thymus-dependent (TD) antigens, and to evaluate the influence that the first antigenic challenge during early life could have in further immune responses provoked a long time afterwards, when individuals were old. We have used the polysaccharide dextran (Dx) B512 as model antigen. Dx B512, extracted from the bacteria Leuconostoc mesenteroides, is a linear molecule composed of glucose units linked in the {alpha}(16) position (14). In its native form (40–100x106 mol. wt), Dx behaves as a typical TI type 2 (TI-2) antigen, inducing polyclonal activation of B cells (1518), and does not induce a memory response. The capacity of Dx to induce a restricted response (19), and the possibility to study a TI (using native dextran) and a TD (with low mol. wt Dx conjugated to a carrier protein) immune response to the same antigen, were some of the reasons for choosing Dx as a model antigen.

As adjuvant we use cholera toxin (CT) from Vibrio cholerae not only because CT has been proven to be potent adjuvant (2024), but also because it allows the delivery of the antigen in soluble form without the inconveniences of emulsion and precipitation that usually mask the real half-life of the antigen since they interfere with the physiological metabolism.

We show here that the type of immune response that the priming event triggers in young life strongly affects the development and maintenance of further specific responses, even if posterior challenges occur a long time afterwards. This immune memory can be positive (when TD antigen is used in early ages) or negative (when it is used as TI antigen) for the host, a fact that has important implications for vaccination.


    Methods
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Animals
C57BL/6 were purchased from Charles River (Uppsala, Sweden) and maintained in our animal facilities at Stockholm University. The animals were 10 weeks old when the experiments started.

Antigens
As a TI-2 antigen, we used native Dx B512 with a mol. wt of 5–40x106 (INC Pharmaceuticals, Cleveland, OH). To obtain the TD protein-Dx conjugate, Dx with a mol. wt of 103 (3–5 glucose units) was conjugated to the protein chicken serum albumin (CSA) (Sigma, St Louis, MO). Dx was conjugated to hydrazide-modified CSA via its terminal aldehyde group using reductive amination (25) and was kindly provided by Dr Christian Krog-Jensen.

Immunizations
Mice were immunized i.p. with the different antigens using the protocols described in Table 1Go. Native Dx, 10 µg/mouse, was administered i.p. in saline with or without the adjuvant CT (List Biological, Campbell, CA), 1 µg per dose. CSA–Dx, 100 µg/mouse, was administered in saline together with CT. Mice were bled by retro-orbital puncture under light anesthesia 8–10 days after all immunizations. In the groups of mice primary immunized at the age of 2–3 months (primed young), we performed two extra bleedings 50 and 310 days after primary immunization. The second immunization was done after a resting period of 10 months (Table 1Go). The resting periods between the immunizations in old mice were 4–8 weeks (see Table 1Go). Serum was separated after centrifugation and serum pools from three to seven mice per group or individual serum was tested in ELISA. To eliminate residual Dextran, some mice (see Table 1Go) received an optimal dose (10 U) of dextranase (Sigma) 30 days after the primary immunization.


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Table 1. Immunization protocol
 
Detection of anti-Dx and anti-CSA antibodies in serum with ELISA
The ELISA was performed as described (24). Briefly, ELISA plates (Costar, Cambridge, MA) were coated with 10 µg/ml Dx T250 (Pharmacia, Uppsala, Sweden) or 10 µg/ml CSA (Sigma). Test sera were added in 2-fold dilutions, starting with 1/200 dilution. Dx-specific mouse mAb (IgM or IgG isotypes) with known concentrations were used as positive controls for the Dx ELISAs. Bound Ig was detected with alkaline phosphatase-labeled goat anti-mouse IgM, IgG, IgG1 and IgG3 (Southern Biotechnology Associates, Birmingham, AL) and p-nitrophenyl phosphatase substrate (Sigma). OD values at 405 nm were determined using an Anthos Reader 2001 (Anthos Labtech Instruments, Saltzburg, Austria). In all ELISAs, as negative control, we used the serum from non-immunized mice of the same age as the mice under study.

Determination of the apparent association constant (aKa)
The principle for measuring affinity of antibodies, in terms of apparent association constant (aKa), by inhibition of binding with free ligand has been described elsewhere (24,26,27). Briefly, the aKa value is defined as the reciprocal of the free ligand concentration that causes 50% inhibition of antibody binding to the plate. As inhibitor we used Dx T40 of mol. wt 4x104 (Pharmacia). Plates were coated with 10 µg/ml Dx T250. The inhibitor was used at concentrations from 10–3 to 10–8 M. Sera were added at different dilutions, ranging from 1/400 to 1/1200. Dx-specific mouse mAb (IgM or IgG isotypes) with known concentrations were used as controls in every plate. Bound Ig were detected with alkaline phosphatase-labeled rabbit anti-mouse IgM, IgG or total Ig (Dakopatts, Glostrup, Denmark) and p-nitrophenyl phosphatase substrate (Sigma). OD values at 405 nm were determined using an Anthos Reader 2001.

Preparation of splenic sections and in situ immunofluorescence
Spleens were removed after tertiary or quaternary immunizations. After removal, spleens were immediately frozen in liquid nitrogen and stored at –70°C. Spleens were embedded in Tissue Tek OCT compound (Miles, Elkhart, IN) and cryostat sections (6 µm) were cut and mounted. The slides were air-dried for 30–60 min and stored at –70°C until use. Cryostat sections were fixed for 15 min in ice-cold acetone. Subsequently, slides were rinsed with PBS and blocked with horse serum (5% in PBS) for 30 min. Sections were stained with FITC-conjugated Dx (FITC–Dx) 250 kDa (purchased from Pharmacia or Sigma) and biotin-conjugated peanut agglutinin (biotin–PNA) developed with streptavidin–Texas Red (TR) conjugate (Vector, Burlingame, CA). This double-staining has been shown to specifically detect Dx binding B cells, by analysis of PNA/FITC–Dx double stained spleen cells from Dx-immunized mice with multiparameter FACS (28,29). Dextran deposits in the sections were detected by protein G-purified anti-Dx mAb 11-1A6 (30) that was conjugated using biotin-NHS-ester (Sigma). The sections were incubated with biotinylated reagents as indicated for 60 min and then incubated with fluoresceinated agents and avidin-conjugated TR for 60 min with washings after each staining. All stainings were performed in a humidified chamber protected from light.


    Results
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Humoral immune response to the TI Dx B512 antigen in old mice: anti-Dx-specific IgM response
To investigate the influence of age in B cell responses, we immunized young and old mice with the native form of Dx B512 to deliver a TI immune response. Young adult mice, 10 weeks old, were immunized with native Dx in the presence (group A) or in the absence of cholera toxin, CT (group B) or left untreated (group E) (see Table 1Go for details). The animals were bled 8, 50 and 310 days after the first challenge, and ~1 year after the first immunization, all animals were immunized again 3 times with TI Dx with or without CT and bled 8 days after each immunization.

In Fig. 1Go, the results show that the magnitude of the primary IgM response induced by TI Dx is very similar in young (80 days old) and in aging mice (392 days old). In the absence of CT as adjuvant, secondary and further immunizations with TI Dx did not increase the response in any of the groups, neither in the group of mice primed with Dx when they were young (group B) nor in those unprimed (group E).



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Fig. 1. TI responses: the B cell compartment is not greatly affected by age. IgM responses (µg/ml) of old mice immunized at a young age with native Dx in the presence (group A) or absence (group B) of CT were compared to the IgM responses of old non-primed mice (group E) immunized only with native Dx. The Dx-primed young animals received 10 U dextranase 30 days after priming. In the x-axis the age of the animals at the times of bleeding is represented in days (7 days after each immunization, except after the primary immunization of primed mice, where the bleeding was done at day 10). In the case of mice immunized early in life, we did two extra bleedings at 50 and 310 days after primary immunization (120 and 380 days of age respectively). Arrows under the x-axis show the point of antigen administration. Means of two independent experiments are shown. Six or seven animals were tested in each group.

 
One important finding is that CT seems to be a powerful adjuvant for TI responses also in old mice (group A), but the effect is only visible after secondary immunization (mice 392 days old), and the levels of IgM increase further after tertiary and quaternary immune responses (mice 427 and 497 days old respectively, group A in Fig. 1Go), which add further support to the notion that B cell responsiveness is not decreased with age.

We have also studied the IgG responses in all groups of mice after immunization with native Dx, but the levels were very low after all immunizations (data not shown), which is what is expected in a normal humoral immune response to native Dx.

Germinal center (GC) formation induced by native Dx (TI) in old mice
We and others have shown that Dx and other TI antigens are able to induce the formation of GC in mice (28,29,31), but that a GC reaction was dependent on T cells (32). In order to elucidate the effect of age in the GC response to TI Dx we immunized old mice of 14–15 months of age with native Dx given together with CT (group E, see Table 1Go). Spleens were taken 10 days after the last immunization, and splenic sections were stained as indicated in Methods and compared with those obtained from young mice (10 weeks of age) immunized in identical form.

As it is shown in Fig. 2Go, we found that the total numbers of GC (measured by the staining of the spleen sections with the lectin PNA, Fig. 2Go, upper photos) was not particularly influenced by age, but that the content of Dx-specific B cells (measured by the staining of Dx–FITC of the same spleen sections) was reduced in old mice (Fig. 2Go, lower photos). Thus, the specific GC reaction is partially affected by age, which is compatible with the idea that the B cell compartment is not affected by age to the same degree as the T cell compartment is. This could mean that the GC reaction, dependent on T cells, is impaired while the T cell-independent humoral immune response is affected to a lower degree.



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Fig. 2. GC responses to TI Dx in old mice. Spleen sections from mice immunized with Dx and CT, taken on day 7 after secondary (young mice) or tertiary immunization (old mice primed with Dx, group A). The sections were stained with PNA (red) or with Dx–FITC (green) to visualize the GC and the Dx-specific B cells respectively.

 
Humoral immune response to the TD form of Dx in old mice
To assess the role of age on the T cell compartment, we used the conjugate CSA–Dx, which we have previously shown in young mice induces an optimal TD immune response against Dx and against CSA with high levels of specific IgG1 in secondary responses to both epitopes (30).

Young adult mice, 10 weeks old, were immunized with CSA–Dx plus CT (group D) or left untreated (group F) (see Table 1Go for details). Approximately 1 year after the first immunization, both groups were immunized with CSA–Dx plus CT. The response against Dx (Fig. 3Go) and against CSA (Fig. 4Go) was measured in the serum from mice bled 8 days after each further immunization. Thus, both groups of mice were old when they were studied, but the only difference is that group D had been primed with a TD form of Dx when young.



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Fig. 3. TD responses: age-associated impairment of specific IgG. Positive effect of priming in early life. Anti-Dx IgM (upper panel) and anti-Dx IgG (lower panel) responses of old mice primed with CSA–DX when young (group D) was compared to anti-Dx IgM responses of old mice non-primed in early ages (group F). Both groups were immunized with CSA–Dx in the presence of CT. Means of two independent experiments are shown. Note the different scales used for the y-axis in upper and lower panels. In the x-axis the age of the animals at the moment of bleeding is represented in days, which was 7 days after each immunization, except the primary immunization of primed mice, for which bleeding was done at day 10. Arrows under the x-axis show the point of antigen administration. Late primary responses at days 120 and 390 for primed mice were 8.6 ± 1.8 and 3.1 ± 1.2 µg/ml respectively in the case of IgM, and 1.5 ± 0.3 and 0.4 ± 0.02 µg/ml respectively in the case of IgG. Six or seven animals were tested in each group.

 


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Fig. 4. TD responses: age-associated impairment of specific anti-CSA IgG. Positive effect of priming in early life. Anti-CSA IgM (upper panel) and anti-CSA IgG (lower panel) antibody responses after immunization with CSA–Dx + CT at 392 days. OD (405 nm) obtained in each sera dilution by ELISA is shown. Group D (primed with CSA–Dx + CT at 10 weeks old) and group F (non-primed mice at early ages) are compared. In the case of IgM, one representative experiment is shown, while for IgG the mean of two independent experiments is shown. Six or seven animals were tested in each group.

 
Both IgM- and IgG-specific antibodies were determined. No major differences were observed in the IgM fraction (Fig. 3Go, upper panel) between the two groups of mice. Unprimed mice (group F) had even a better secondary immune response than those primed when young (group D). However, the IgG response was clearly affected by age (Fig. 3Go, lower panel). Apparently, old mice that were not primed at an early age with TD Dx–CSA (group F) are unable to mount and maintain an appropriate IgG response to Dx, which is in agreement with T cell responses, and consequently IgG, being impaired with age (33,34).

However, and most interesting, priming of mice when they were young with TD Dx–CSA did prevent the negative effect of age in the response to the same antigen, because the specific anti-Dx IgG response is maintained and increased (group D at days 427 and 497, Fig. 3Go, lower panel), which is the common pattern of response to TD antigens in immunologically healthy young adults. Similar results were observed for CSA epitopes in the CSA–Dx conjugate (Fig. 4Go).

Moreover, the group of primed mice in early ages (group D) showed a 10 times higher anti-Dx IgG response at 56 weeks of age compared to the response of non-primed mice (group F) at the same moment (Fig. 3Go, lower panel). Thus, a single immunization with CSA–Dx induces a memory state lasting for >1 year which is remarkable. This long-lasting memory was also observed in the anti-CSA IgG response (Fig. 4Go, lower panel).

TI antigen immunization in young adult life leads to a suppression of TD responses in the elderly
The hallmark of the immune response is the induction of immunological memory which is frequently interpreted in a positive manner. However, we have previously shown that priming with the TI antigen form of Dx could provoke a state of unresponsiveness in the specific IgG to Dx when mice were later challenged with an optimal TD Dx–protein conjugate (35). We now wanted to investigate if this `negative memory' was as long lasting as the TD induced memory. To assess this, we primed young adult mice with native Dx (TI antigen) in the presence of CT and we boosted the animals with CSA–Dx (TD antigen) ~1 year later (group C). We compared the IgM and IgG responses to CSA–Dx of these old mice with the responses of old mice that were not primed with TI antigen when young (group F). As expected from our previous observations (35), the IgM responses were of similar magnitude in both groups (TI primed and in the non-primed mice, data not shown).

However, it is worthy to note that there were clear differences in the IgG Dx-specific responses, the response in the TI primed group being ~30% lower than the response in the control group (Fig. 5Go, upper panel). Thus, our results show two interesting points: (i) the TI (Dx native) priming when animals are young has a negative effect on further TD responses to Dx–CSA and (ii) TI priming has a long-lasting negative effect on those TD responses (at least 1 year in the mice).



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Fig. 5. Inhibition of specific anti-Dx IgG in the elderly as a consequence of TI priming in the young. Upper panel: anti-Dx IgG responses (µg/ml) after immunization with CSA–Dx (TD antigen) of mice primed early in life with native Dx (TI antigen) in the presence of CT (group C) and the IgG response in non-primed mice (group F). Arrows indicate the points of antigen administration. The Dx-primed young animals received 10 U dextranase 30 days after priming. Lower panel: anti-CSA IgG responses at 427 days of primed mice primary immunized with native Dx (TI antigen) in the presence of CT (group C in Table 1Go), and the IgG response in non-primed mice (group F in Table 1Go). Means of two independent experiments are shown. Six or seven animals were tested in each group.

 
A decrease in the response to CSA was also observed in mice primed with native Dx at an early age (group C in Fig. 5Go, lower panel), but the reduction was much smaller than that observed with the Dx epitope (Fig. 5Go, upper panel).

Finally, we analyzed the pattern of expression of specific anti-Dx IgG1 versus IgG3 responses to assess whether priming with the TI antigen could have a regulatory effect, i.e. on the switch from a TD IgG such as IgG1 to a more TI pattern such as IgG3. We did not find any change in the pattern of IgG production, both groups of mice a having complete absence of IgG3 responses (data not shown).

The affinity of anti-Dx antibodies in old mice
To completely understand the immunological status in the old mice, we studied the relative affinity of the anti-Dx IgM and IgG responses by hapten inhibition as described in Methods.

As expected, we could not find any important difference in the relative affinity of the IgM responses (Table 2Go). The age, antigen or the number of immunizations did not significantly influence the affinity of the Dx-specific responses in any of the groups analyzed. This could indicate that age does not affect much the response to TI antigens.


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Table 2. Relative affinity of specific anti-Dx IgM responses
 
In contrast, the affinity of the IgG response was influenced by various factors. The TI immunization in young adult life followed by TD immunization 1 year later (group C) caused a decrease in anti-Dx IgG affinity (Table 3Go). The relative affinity of this group of mice at 427 days is lower than in old mice primed or non-primed with TD Dx–CSA when they were young (groups D and F respectively, Table 3Go).


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Table 3. Relative affinity of specific anti-Dx TD IgG responses
 
These results suggest that the TI priming in early life affects not only the magnitude but also to the quality of the IgG response. Similarly, priming in early life also increased, besides the magnitude, the affinity of the response in the old. The IgG response at 392 days in group D (mice primed when young with TD) is of higher affinity than the IgG of group F (non-primed). Thus, the `positive' (TD antigen CSA–Dx) and the `negative' (TI antigen Dx) priming in early life influences the affinity of the IgG response.


    Discussion
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Immunosenescence is considered to be caused by a dysregulation and deterioration of the immune functions, manifested as an increased susceptibility to infectious diseases, autoimmune diseases and even cancer (1). Many aspects related to elements or events within the immune system such as changes in the B and T cell compartments and defective GC reactions have been implicated in the process of immunological aging. The intention of the present work was to improve our knowledge of aging by taking advantage of a system that allows us to separately study the immune status of the T and the B cell compartments. For this purpose we have used the carbohydrate Dx B512 in a TD (25,30,36) and a TI form (15,16), and the potent adjuvant cholera toxin (CT) (20,21).

Our results agree with reports from other groups (6) suggesting that the B cell compartment is not affected by age as strongly as the T cell compartment. We have observed only a minor decline in the IgM responses to both the TI and the TD Dx, and a lower efficiency of the GC reaction (Fig. 2Go). We also show here that CT can be an effective adjuvant in aged mice, increasing the humoral responses to Dx, suggesting that B cells do not loose the ability to respond to the adjuvant effect of CT with age (Fig. 1Go, group A).

Contrary to the B cell responses, the responses to TD Dx were affected by age. While the specific IgG response in young adult mice increases in magnitude and in affinity after each antigenic challenge, this is not the case in the aged (group F in Fig. 3Go and Table 2Go). Old mice, that did not receive any previous immunization, have an altered immune response to CSA–Dx, with lower levels of IgG, especially to Dx (group F in Fig. 3Go, lower panel), and to a lower degree to the carrier CSA (group F in Fig. 4Go, lower panel).

However, even if the system is aged, the specific memory response in the elderly can be improved if those mice are primed with the TD antigen when young (group D in Fig. 3Go, lower panel, and Table 2Go). This underlines the essential role of the priming event early in life. Apparently, the time point for the stimulation is of importance. Stimulation of `young' immune cells when they are fully competent provides these cells with the capacity to be better stimulated in an aged system. Thus, the young priming (vaccination with TD antigens) appears then as a way to `correct' the deficits in the aged immune response, providing a `positive memory'.

An important observation is that a single injection with a TD antigen was sufficient to maintain the memory state for >1 year. Long-lasting memory has exclusively been seen to occur after viral infections, repeated immunization with particulate TD antigens, or with antigens co-precipitated with alum or emulsified in Freund's adjuvant. In all these cases, the persistence of antigen has been proposed to contribute to the maintenance of memory (reviewed in 37), but in our experiments the antigen is administered with CT in a soluble and easily degradable form. Moreover, we treated the mice with the Dx-specific enzyme dextranase to facilitate the destruction of the antigen. Whether memory is maintained due to (i) the existence of long-lived B and/or T cells, (ii) the persistence of soluble antigen in the mice 1 year after a single administration, or (iii) a CT-mediated facilitation of the memory process are important questions, but cannot be answered with the data presented here.

Equally important to promote a correct immunogenic challenge would be to avoid an inappropriate encounter with the antigen. As shown here (Fig. 5Go) and elsewhere (35), a primary immunization with TI Dx abrogates the specific IgG response to posterior challenges with the TD antigen both in magnitude and in affinity (group C in Fig. 5Go and Table 3Go). Moreover, this state of unresponsiveness or `negative memory' is apparently very long-lasting (at least >1 year). Thus, our results suggest that the immunization with TI antigens (e.g. vaccination to pneumococcal carbohydrates) in early ages may not be a convenient strategy because further immune responses to the same antigen could be inhibited.

Several explanations of this negative priming with TI antigens are compatible with our results such as the presence of specific or anti-idiotypic antibodies able to block the antigen or the antibody respectively (3841) and antibodies producing a negative feedback through interaction with the Fc{gamma}RIIB receptors on naive B cells (42) by the phenomenon named `original antigenic sin' (43,44). Both explanations are unlikely because we did not find specific antibodies 310 days after the primary response (group C in Table 3Go) and the antibody response induced by Dx is mainly of the IgM class which does not bind to the Fc{gamma}RIIB receptors on naive B cells. Other more trivial explanations related to a modulation of the kinetics in TI-immunized mice can also be discarded. This issue was studied in detail before (35). The reduction of the IgG was found to be permanent.

Other explanations related to the oligoclonality of the response to Dx can also be considered. Immunization with TI Dx induces a VH/V{kappa}-restricted response that occurs late in ontogeny (19,39,41,45). It is possible that this oligoclonal repertoire to Dx in the absence of memory could disappear by clonal exhaustion and in consequence a secondary challenge even by a TD antigen would not be able to induce an optimal response.

The conclusion of our work is summarized in Fig. 6Go. To our knowledge, this is the first experimental demonstration showing that antigenic challenge early in life has a profound influence in the subsequent immune responses and that this influence can last during the whole life of the host. Therefore, the age-associated impairment of the immune response can be, at least in part, corrected in early life, a fact that changes the concept of `immunosenescence'. Age provokes defects in the immune responses, but the way in which the young immune system encounters the antigen for the first time (as TD or TI antigen) has a critical role in producing these defects, which has relevant implications in vaccination. While early TD priming increases late TD responses in terms of both quantity and affinity of antibody responses (`positive memory' in Fig. 6Go), the early priming with TI antigen inhibits late TD responses to the same antigen (`negative memory' in Fig. 6Go).



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Fig. 6. Positive and negative memory.

 
Thus, new strategies have to be explored such to attempt an specific rejuvenation of the immune system by priming at an adequate time point and with the appropriate antigenic substance (a TD antigen form). This would help in the design of vaccination programs aimed to prevent or ameliorate immunosenescence.


    Acknowledgments
 
This work was supported by grants from Medicinska forskningsrådet (MFR), Sweden and from Fondo de Investigación sanitaria (FIS) 99/1170 Ministerio de Sanidad y Consumo, Spain. M. S. C. and K. L. were supported by personal fellowships from Fundación Ramón Areces (Spain) and from the Stockholm University (Sweden) respectively.


    Abbreviations
 
aKa apparent association constant
CSA chicken serum albumin
CT cholera toxin
Dx dextran
GC germinal center
PNA peanut agglutinin
TD thymus-dependent antigen
TI thymus-independent antigen
TR Texas Red

    Notes
 
Transmitting editor: C. Martinez-Alonso

Received 18 June 2001, accepted 19 June 2001.


    References
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

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