Immunological relationships during primary infection with Heligmosomoides polygyrus (Nematospiroides dubius): parasite specific IgG1 antibody responses and primary response phenotype

IgG1 antibody responses to Heligmosomoides polygyrus were measured in eight mouse strains supporting acute (< 8 weeks, SJL, SWR), intermediate (10–20 weeks, NIH, BALB/c) or chronic (> 25 weeks, C57BL/0, CBA, C3H, AKR) primary infections. Mice supporting acute or intermediate infections produced more intense antibody responses and total serum IgG1 concentrations were higher than in mice tolerating chronic infections. Positive correlations across mouse strains between the intensity of the antibody response and the percentage loss of worms in weeks 6 and 10 were established. No correlation was found between the response within mouse strains and loss of worms by individual mice. Heavy infections gave marginally higher antibody titres than low intensity infections, but few significant differences were detected and it was concluded that infection intensity did not markedly influence the magnitude of the antibody response. Male and female mice responded similarly despite the earlier loss of worms from females. No association was found between the primary response phenotype and recognition of particular antigens in Western blot analysis, nor did intensity of infection or host gender affect recognition. The possibility that immunomodulatory properties of adult worms may have had a differential influence on ability of strains of contrasting response phenotype to mount IgG1 responses was discussed.


INTRODUCTION
Parasitic nematodes responsible for chronic infections, are of considerable importance both in human and veterinary medicine (See reviews Behnke 1987, Behnke, Barnard & Wakelin 1992a, Monroy & Enriquez 1992. Despite their economic and medical significance, the mechanisms exploited by nematodes in surviving within otherwise immunocompetent hosts are still poorly understood.
Heligmosomoides polygyrus is a murine intestinal species which has been studied in an effort to determine the factors enabling chronic infections to persist (Behnke 1987, Monroy & Enriquez 1992. Following primary exposure many mouse strains harbour adult worms for more than 30 and often up to 40 weeks , whereas others curtail primary infections within eight weeks of exposure , an observation which is of particular significance because it implies that certain mouse genotypes are resistant to the evasive strategies employed successfully by adult H. polygyrus during chronic infections in susceptible strains. Although the immunological processes accompanying loss of H. polygyrus during primary infections are not understood H . polygyrus is known to be susceptible to expulsion in mice in which intestinal inflammation has been induced non-specifically (Behnke, Cabaj & Wakelin 1992b). Recently Urban, Katona & Finkelman (1991) have produced data indicating that in BALB/c mice (Intermediate responders), CD4+ cells are required to control primary infections and that depletion of these cells through treatment with specific antibody results in a depression of serum IgE and an increase in survival and fecundity of adult worms. Collectively these data suggest that some mouse strains can reject the worm through the mediation of CD4+, Th2 lymphocytes which are known to control inflammatory responses to other G1 nematodes (Finkelman et al. 199 1 a, Grencis, Hultner & Else 1991.
The data necessary to substantiate this hypothesis are difficult to obtain because of the long time frame over which relevant experiments have to be conducted and a role for Th2 cells can be questioned because even fast responding strains fail to show a prominent mastocytosis during primary infections (Dehlawi, Wakelin & Behnke or were bred in the departmental animal house under conventional conditions. All animals were provided with food and water ad libitum.

1987). On the Other hand H . pol%Yrus elicits prominent
The parasite used in this work was He~igmosomo;des parasite specific responses and these depend On bakeri (Behnke,Kcymer & Lewis 199 1). The help provided by Th2 (Stevens et 19887 Snapper, employed for maintenance, infection of mice Finkelman 19883 Mosmann ' 'Offman 1989)' Originally IL-4 was considered to be necessary for lgGl responses but the recent demonstration that in vivo depletion of 1L-4 abrogated IgE responses but did not and recovery of worms at autopsy have all been described previously (Jenkins & Behnke 1977).
affect IgGl implies that other Th2 cytokines must be involved (Finkelman et al. 1991).
lgGl clearly has an important role to play in the hostparasite relationship of H . polygyrus although the involvement of IgGl in mediating host protective immunity during primary infections is still debatable. Whilst some studies have shown that primary infection IgGl neither mediates nor blocks protective immunity , Pritchard, Behnke & Williams 1984 others have attributed a protective function to this isotype (Dobson 1982). Since detailed information on the factors determining the intensity, rapidity and specificity of IgG1 responses to H . polygyrus and the relevance of these to worm survival are still lacking, we report here the results of a series of experiments which focus specifically on this aspect of the host-parasite relationship. These experiments examined the hypothesis that an inverse relationship between the duration of primary infection and parasite specific IgG 1 responses should exist across strains supporting varying durations of infection, if lgGl responses constiute or accompany an essential host component limiting the survival of adult worms. In addition we have correlated IgGl responses and the duration of adult worm survival among individual mice within strains and compared both low and high intensity infections and infections in male and female mice within strains. Both sets of variables have a marked influence on the duration of infection ) and the intensity and rapidity of IgG1 responses should therefore vary accordingly.

Preparation of antigens
Outbred CFLP mice were infected with 400 L3 of H . polygyrus and were killed 14 days later. Adult worms were isolated by opening the small intestine and incubating sections harbouring worms in Hanks's saline at 37°C in gauze bags held over small glass beakers. When sufficient numbers of worms had collected, the parasites were washed ten times in ice cold sterile phosphate-buffered saline (PBS) and were homogenized in minimal volume of PBS using a glass tissue homogenizer held in an ice bath. The resulting suspension was centrifuged at 10000 g for one h at 4°C to remove coarse particulate matter. The supernatant was filtered (0.22 pm filter, Millipore), analysed for protein concentration using a method modified from Lowry et ul. ( 195 I), aliquoted and stored at -40°C.

Measurement of antibody responses
Specific anti-worm antibodies were measured by a standard ELISA but the data for each experimental group are presented as a mean relative response index (RRI). This was necessary to enable large numbers of sera to be screened simultaneously and to enable comparison of values from different experiments. Briefly, ELISA microtitre plates were coated with 50 pl/well of worm antigen (5 p g ml-I). Alkaline phosphatase conjugated sheep-antimouse IgGl (Serotec) was used to measure subclass specific responses. After addition of the substrate (P-nitrophenylphosphatase) colour changes were read at 410 nm on a Dynatech MR700 Microplate Reader. Sera were assayed individually in triplicate after storage at -40°C and a mean optical density (OD) value was obtained for each serum. Each plate included, control hyperimmune serum (HIM) which was raised as described by Behnke & Parish (1979) and control serum from naive mice and these were used to calculate the RRT for individual sera using thc following formula: Thus the mean OD for individual sera was expressed as a percentage of the OD obtained with the hyperimmune reference serum, after subtraction of the value for nai've control serum. RRIs measured on individual sera on different occasions showed little variation and the results were consistently reproducible. The same stocks of aliquoted HIM and control naive serum were used throughout the study. Group mean RRls were calculated from individual RRIs. Total serum IgGl in the sera from individual mice was determined by the method of Mancini, Carbonara & Heremans (I 965) using radial immunodiffusion (RID) kits purchased from The Binding Site, Birmingham. Ring diameters were measured in two directions at 90" and the mean was used to calculate the concentration of immunoglobulins from a calibration curve obtained using appropriate standards.
The spectrum of antigens recognized by antibodies in the sera of infected mice was examined by the Western blotting technique essentially as described by Carr & Pritchard (1 987). Phosphate-buffered saline (PBS) soluble adult worm homogenate was electrophoretically transferred to nitrocellulose membranes (Schleicher and Scuell) following separation on 5-20% gradient SDS-PAGE. Strips of membrane were blocked with 5% skimmed milk in PBS-Tween 20 (0.05%) and then incubated overnight at 4°C with appropriate test sera (1 : 100) using 5% skimmed milk in PBS-Tween 20. After washing in PBS-Tween 20 the strips were incubated with peroxidase conjugated sheep anti-mouse IgGAM or IgGl (Serotec) or 12SI-sheep anti-mouse polyvalent immunoglobulins (0.5 pCi/strip, Amersham, Bucks, UK) for 2 h at room temperature. Following washing in PBS-Tween 20 (3 x 20 min) the strips were immersed in hydrogen peroxide and 4-chloro-l naphthol chromogen until the reaction developed optimally, rinsed, arranged on a suitable black background and photographed while still wet. In the case of the 1251-labelled IgGAM, the strips were air dried, fixed to the interior of an X-ray cassette and exposed to an X-ray film (Fuji) at -80°C in the presence of an intensifying screen. The molecular weights were estimated with reference to the migration of standard molecular weight markers.

Experimental design
For logistic reasons, it was not possible to conduct large scale experiments with comprehensive comparisons  Weeks after infection among all the relevant mouse strains. Combinations of mouse strains were studied in a series of 24 experiments, involving, at most, four strains per experiment. Each experiment focused on particular aspects of the host response and therefore sufficient material was not always available to complete all the possible analyses. However, as far as possible most were repeated, sometimes additional data becoming available through inclusion of necessary controls in experiments investigating other aspects of infection. Table 1 presents a list of the experiments, specifying the strain and sex of mice involved and the infection intensity administered.

Statistical analysis of results
Antibody responses are presented as group mean RRI value + standard error (SEM). Non-parametric statistical procedures were used to analyse the data sets, because normal distribution of data could not be assumed (Sokal & Rohlf 1969). When more than two groups required comparison at a single time point the Kruskal Wallis statistic H was calculated to determine whether there was a strain effect across experimental groups. If significant, specific groups were compared to the control group (or as stated) by the Mann-Whitney I/ test. Correlations between variables were tested by the Spearman Rank Order Correlation Test and the statistic r, is given, as appropriate. Probabilities were calculated from statistics tables and P=0.05 was taken as the cut-off point for significance unless multiple analyses were undertaken when the cut off value for significance was lowered to P = 0.0 I in order to avoid Type I I errors.

The kinetics of low-intensity primary infection in fast, intermediate and slow responder mouse strains
The kinetics of primary inlection with H . polygyrus in mouse strains tolerating long-lasting chronic infections (Robinson et ul. 1989) and those limiting infection earlier (Wahid et ai. 1989) have been described previously. The present work is based on a range of mouse strains representing three primary response phenotypes, slow (resulting in chronic infections > 25 weeks, e.g., C57BL/ 10, CBA, C3H and AKR), intermediate (10-20 weeks, e.g., NIH and BALB/c) and fast (resulting in acute infections < 8 weeks, e.g.. SJL and SWR) and, for reference, representative examples of the time-course of infection in each of these strains are summarized in Figure 1.

Serum IgGl responses to parasite antigens during the course of acute, intermediate and chronic infections
Data from 15 experiments are illustrated in Figure 2, each experiment measuring the response in only one or at most three strains of mice. Mice were infected with 50-60 L3 unless otherwise stated and the infectivity of the inocula used was in the range 54% (Experiment 15) to IOO'X, (Experiment 5), although the poor establishment in Experiment 15 was unusually low. It is evident that the response of C57BL/10 mice was extremely poor, in some experiments virtually no detectable antibody being observed in the first 10 week period of infection (Figures 2a,b,c & e). At most the R R I rose to 25% in this strain (Experiment 3, Figure 2a). Likewise the other strains  Figure 2f, two experiments, not illustrated here, showed much the same pattern. In the first of these (Experiment 11) the RRI in CBA did not rise above 10% during 18 weeks post infection (the response was measured in weeks 2,6,8,14 and 18 in a group of nine female mice infected with 50 L3). In the second (Experiment 17), groups of 10-1 1 CBA and 7-8 C57BL/10 mice were killed 20 and 31 weeks after infection with 50 L3. In week 20 the RRI was 20.3 f 2.6 and 13.5 3.2 for CBA and C57BL/10 mice respectively ( P = NS) and in week 3 1, 28 f 4.9 and 14.4 f 4.3 (0.05 > P > 0.025) respectively.
Both experiments therefore confirm the low intensity IgG 1 response of CBA mice to H . polygyrus infection.
In marked contrast SJL and SWR mice both showed a rapid IgGl response with significantly enhanced levels being detected within the first five-week period of infection. In all direct comparisons SJL and SWR mice responded more vigorously than C57BL/ 10 mice (Figures  2b & c).
Both intermediate responder strains, NIH and BALB/c, also had more intense IgGl responses than the slow responding strains when compared within or across experiments. For example in Experiment 13 the responses of NIH and C57BL/10 mice were compared directly and The symbols used to designate strains have been described in Fig. 1. Additional symbols were also used to tlcnote SWR and SJL strains: the difference in week ten was highly significant (0.01 > P>O.OOI). Likewise in Experiment 21 (data not illustrated) the RRI in week eight was higher in NIH mice (43.6f6.0, n=9) than in C57BL/10 (18.7k4.4, n=8, P = 0.03). In another experiment (Experiment 2 G d a t a not illustrated) BALB/c mice (n = 6) were compared directly to C57BL/IO eight weeks after infection. The RRls were 15.2 f 3.5 and -1.4f 3. I respectively. In this same experiment the RRI of SJL mice (n=6) was 25.7 & 4.6.
The difference between the fast responders (SJL & SWR) and the moderate responders (NIH & BALB/c) was less clear-cut. As can be seen in Figure 2d BALB/c mice showed considerable variation between experiments, on occasion responding as vigorously as SJL mice (Figure 2d, Experiment 9) although when high titres were recorded from this strain it was always several weeks later than in SJL mice. Likewise N IH mice responded variably, often with a peak RRI in excess of 40% (Figure 2e

Relationship between antibody response and worm loss at times following infection
Two types of analyses were undertaken to relate the intensity of antibody responses to loss ofworms. The first Weeks a f t e r infection analysis addressed directly the prediction that there should exist an inverse relationship across mouse strains between the intensity and rapidity of the IgGl responses and the survival of worms. In order to arrive at a single measure of worm expulsion and RRI for each strain, data from relevant experiments were averaged. Figure 3 summarizes the results of this analysis. As can be seen there was a significant negative correlation between the RRI for IgGl and worm survival across the seven mouse strains studied in both week six and week ten.
In the second analysis an attempt was made to correlate the XRI of individual mice within strains with worm survival assessed as the proportion of the group mean worm burden on day 14 (reflecting worm establishment) surviving in individual mouse until the day of autopsy. Correlations between RRI and percent worm survival were attempted for slow, intermediate and fast responder   occurred too quickly for the correlation to be attempted: all the worms were present in the early stages of infection,

Effect of intensity of infection on antibody response
whereas latcr autopsies revealed almost complete expulsion. The results are shown in Table 2. There were no Earlier published data showed that worm loss was dependent on the infection intensity in some strains, notably in BALB/c mice  in which high intensity worm burdens were lost later than low intensity infections. Therefore, it was of interest to examine the relationship between intensity of infection and antibody response to H . polygyrus. Accordingly sera from relevant experiments were analysed for IgGl specific antibody activity. Antibody responses in mice subjected to high and low intensity infections were also studied in BALB/c, NIH, SWR, SJL, C57BL/10 and CBA. The infectivity of the inocula used for these experiments varied in the range 54% (Experiment 15) to 95% (Experiment 10). The results are presented in Figure  4.
Overall there was little difference in the IgGl antibody response between groups given low or high intensity primary infections. Among the intermediate responder strains, BALB/c mice, in which worm loss is significantly accelerated during low intensity infections, averaged higher antibody titres following the more intense exposure although a significant difference was detected on only one occasion (week 15, Figure 4b). NIH behaved similarly (Figure 4d), mice given heavier inocula having higher mean titres but a significant difference was only detected in week 20.
Fast responder SWR mice likewise showed no significant difference (Figure 4f). Interestingly however, SJL mice, in which there appears to be no infection-intensity effects on the timing of expulsion , had higher average titres when exposed to low intensity infections, but these were not significantly different from the group given the higher intensity exposure.

Effect of host gender on antibody response
Following low intensity exposure, female mice generally lose adult H . polygyrus sooner than males . It was of interest therefore, to determine whether host gender-dependent loss of worms would be reflected in differences in antibody activity between the sexes. Figure  Female SWR mice (Experiment 18) expelled worms earlier than males (In week seven females [n=8] had 14.6 f 6.4 worms, males had 34.3 f 6.1 [n = 71, P < 0.05), but as can be seen from Figure 5a, IgGl titres were similar in both sexes on all occasions tested. In Experiment 10 ( Figure 5b) female NIH mice responded faster than males as reflected in antibody titres in week five but at subsequent time points there were no significant differences in antibody titres between the sexes. However, differences between the mean worm burdens did not become apparent until week ten (week ten, males 37.7 f 5.9, females 23.2 & 6.7; week 15, males 20.7 f 7.6, females 10.7 k 4.9; week 20, males 21.3 f 6.1, females (6.7f2.9) although these were not significant because of variation in parasite burdens between individuals in each group. In a second comparison involving NIH mice (Experiment 21, data not illustrated) there was a significant difference (P=0.004) in worm burden in week eight (females 21.7 f 3.7 and males 34.4 f 3.7) and the mean RRI of female mice was also higher (females 43.6f 6.0, males 25.3 f 6.8) but not significantly so. Figure 5 also illustrates results from Experiment 17 in which strains representing the slow responders were studied. Female CBA mice had significantly higher titres than males in week 31 (Figure 5d) but there was no difference in the response of C57BL/10 mice. There were no significant differences in worm burden in either strain (data not shown).

Comparison of total serum IgGl during low-intensity infections in mice of varying responder phenotype
Total serum IgGl was measured in selected strains representing the three categories of responder phenotype. Data from 5 experiments allowed such comparisons. The results are summarized in Table 3. Although it was not possible to measure IgGl concentrations in naive mice in all experiments, the data which were obtained show that 1-2.5 mg/ml can be expected in uninfected mice and this is consistent with other reports in the literature (Crandall, Crandall & Franco 1974, Pritchard, Behnke & Williams 1984. With this baseline it is quite evident that the slow responder strains had relatively modest changes in total serum IgGl during the course of the first ten weeks following infection, maximal levels not exceeding 4 mg/ml. In contrast both intermediate and fast responder strains showed greater changes with IgGl serum levels rising to 5-10 mg/ml.

Comparison of antigen recognition during low-intensity infections in mice of varying responder phenotype
Sera from the experiments described above were used to compare antigen recognition profiles on Western blot analysis. A large number of gels were run in total. Each gel examined 12-24 individual sera and each was controlled with a pool of naive sera and a pool of hyperimmune sera at one extremity. Comparisons were made using the sera of individual mice within strains, before worm loss and after loss of worms. Additional comparisons were made across strains, between sexes, and between high and low intensity exposed mice.
After compensation for differences in overall antibody titres of sera under comparison, none of the analyses undertaken revealed any major differences in the reactivity of sera to parasite antigens. There was some variation between individual animals within experimental groups comprising particular strains but no consistent differences which could be attributed to strain specific responses to particular antigens or within strains to the timing of worm loss. Hyperimmune sera consistently showed up three bands of low-molecular weight when worm homogenate antigen was subjected to Western blot analysis. These resolved at 16, 18 and 20 kDa. Only the lower of these bands was identified by our experimental mice subjected to primary infections and there was variation between sera in its recognition but we could not ascribe this to any of the variables which were of interest in the context of the present paper. C57BL/10 mice never recognized this antigen but BALB/c mice did so although not in a totally predictable manner. BALB/c mice which had been subjected to a low level infection (Experiment 15) and had expelled most of their worms by week I5 did not recognize the 16 kDa antigen. However, some (but not all) mice in Experiment 15, given heavier infections and still harbouring substantial worm burdens in week 15 (1 09.6 5 20.0), did so. In Experiment 19, SJL mouse sera taken immediately after expulsion (week eight) recognized the 16 kDa antigen whilst in other experiments with SJL mice we could not detect reactivity against this antigen.

DISCUSSION
The parasitic nematode H . polygyrus is unusual for the chronicity of the infections which develop after primary exposure of mice to infective larvae (Keymer & Hiorns 1986, Robinson et (11. 1989). On repeated exposure, acquired immunity eventually develops and is known to be associated with specific IgGl antibodies and with an increase in total serum lgGl often exceeding 20 mg/ml (Chapman et al. 1979a. In contrast, primary infections seldom cause more than a two-fold increase in total IgGl (Molinari, Ebersole & Cypress 1978) and specific titres are significantly lower than in hyperimmunized animals (Crandall et al. 1974, Pritchard et al. 1984. Because the involvement of IgGl in determining the duration of primary infections is still controversial, a clarification of the function of this isotype is important if the role of Th 1/Th2 cells is to be understood in relation to parasite evasive strategies. For this reason we analysed sera collected during primary infections in various mouse strains  for antibody responses within this immunoglobulin isotype and we addressed the hypothesis that if IgGl is involved in limiting primary infections, an inverse relationship should exist between the intensity and rapidity of specific IgGl responses and parasite survival. A panel of eight syngeneic mouse strains was employed, representing three responder phenotypes which could be clearly differentiated from each other. Thus chronic infections were sustained in C57BL/10, CBA, C3H and AKR mice, infections of intermediate length (generally 10-20 weeks) in BALB/c and NIH mice and infections never lasted more than eight weeks in SWR and SJL strains. Comparison between strains a t different time points during infection revealed that all the fast responder mice generated parasite specific IgG 1 antibodies during the first five weeks of infection, peaking during the 6-8 weeks when loss of worms was taking place. BALB/c and NIH mice also developed strong responses but these peaked later corresponding with the relatively delayed onset of worm loss. In marked contrast IgGl responses in the slow responder strains were without exception of low intensity, in some experiments with C57BL/10 mice no detectable antibody being evident until well after five weeks of infection. We established an inverse correlation across mouse strains between the intensity of their specific IgGI antibody response to parasite antigens and worm survival (Figure 3). At first sight these data are consistent with the hypothesis that IgGl plays a role in limiting primary infections.
However, other data reported herein are not consistent with this notion. Firstly, there were no significant correlations between the intensity of the specific IgGl response within mouse strains and loss of worms by particular individuals (Table 2), as might be expected were IgGl to play a crucial role in worm expulsion (See Dobson, Brindley & Sitepu 1982). Secondly, in groups of mice subjected to low or heavy infections, among which the former lose worms earlier , it was the more heavily infected animals which had marginally higher IgGl titres (See Dobson & Cayzer 1982). Thirdly, there was no significant difference between the sexes in the IgGl response, despite the earlier loss of worms from female BALB/c and NIH mice in these experiments. The greater resistance of female mice to infection is well documented in the literature (Dobson 1961, Dobson & Owen 1978 but, in contrast to our findings, more intense antibody responses have been observed in female relative to male mice by other workers (Dobson & Cayzer 1982, Dobson 1982. Lastly we could not detect any difference in antigen recognition profiles which might have indicated that particular mouse strains limited infection through their ability to identify specific antigens. Essentially all strains responded to the same antigens with only minor variations between individuals within strains and a suggestion that some differences existed in respect of recognition of the 16 kDa antigen. There may be a simple explanation for all of the above findings. The data in Table 3 show that mice which ranked as slow responders produced only modest changes in their total IgGl serum levels and it is therefore possible that mice which produce strong specific IgGl responses to H . polygyrus may be doing so because they are good IgGl responders and not because it is a component of their protective response to parasite infection. Our results may simply reflect a secondary association arising from a common mechanism controlling both protective responses to H . polygyrus and igG 1 secretion. However, to our knowledge, C57BL/10, CBA and C3H mice are not intrinsically poor producers of IgGl to other antigens and parasites (Chapman et al. 1979b, Lebrun & Spiegelberg 1987, Delgado & McLaren 1990, Lopes et al. 1990). AKR mice were found to respond weakly with IgG1 relative to B 1 OBR mice following infection with Trichinella spiralis (Pond, Wassom & Hayes 1989) but were later reported to generate mainly IgG1 and IgM responses to group I1 antigens from the L1 stage of this parasite (Denkers et al. 1990). We therefore consider it unlikely that the poor IgGl responses of the slow responder strains can be attributed solely to a genetically determined constraint on IgG 1 responsiveness to relevant antigens.
An alternative possibility is that H . polygyrus actively interferes with host IgGl responses to infection, perhaps as a consequence, or an essential component, of the immunomodulatory strategy which adult worms employ to enable their chronic survival (Lossom, Lloyd & Soulsby 1985, Dobson & Cayzer 1982, Behnke, Hannah & Pritchard 1983. Parasite immunomodulatory factors (IMF) may downregulate IgG1 responses more effectively in mice of particular genotypes (Else & Wakelin 1990). The poor host-protective responses of C57BL/10 and CBA mice to H . polygyrus have been reported previously (Ali & Behnker 1985, Parker & Inchley 1990) and we have hypothesized that IMFs may be more effective in mice bearing the H-2b and H-2k haplotypes associated respectively with these strains (Behnke & Wahid 1991). Both mastocytosis and IgGl responses appear to be suppressed in slow responder mice (Dehlawi et al. 1987), whilst in moderate responders (H-2d and some H-24, e.g., BALB/c and NIH) and in fast responders (H-2s and some H-2q e.g., SJL and SWR), mastocytosis is downregulated but IgGl responses are less affected. H . polygyrus therefore, may be capable of affecting differentially the products of an initial Th2 response, with a more severe and wide ranging influence in slow responders compared with fast responders. Speculatively, this may be achieved by several distinct IMFs some of which target MHC and others cytokine regulatory pathways or by a single factor with pleiotropic properties, some of which are dependent on interactions with MHC. Indeed, it is likely that IMFs may prove to be molecules much like cytokines (Haig & Miller 1990) with equally complex interactions as suggested by Behnke et al. (1 992a).
In conclusion, the experiments described in this paper have clearly demonstrated that mouse strains which speedily limit primary infections with H . polygyrus, produce more intense specific IgG 1 antibody responses to parasite antigens and greater amounts of total serum IgG I than strains which tolerate chronic infections. This relationship held across eight mouse strains representing three clearly distinguishable primary infection phenotypes. However, other data were not compatible with the view that IgGl antibodies mediated protection directly and that worm loss was dependent on the intensity of the specific IgGl response. We suggest that a consequence of the parasite's survival strategy involving IMFs, is more effective downregulation of some mouse genotypes to respond with IgGl than others. In consequence the intensity and rapidity of IgGl responses during primary exposure to H . polygyrus reflect host response phenotype as assessed by the duration of infection.