The results in this paper are from a large number of internally controlled experiments aimed at clarifying how an animal mounts an immune response to a natural infection of its gut epithelium. The infection system under study, coccidiosis, is a protozoan-induced disease that afflicts most vertebrates before and throughout breeding age and as such may have provided an evolutionary pressure on intestinal immune responses. In addition to being economically important in its own right, Eimeria is closely related to several human pathogens that either reside in or enter the body via the intestinal epithelium, e.g., Cryptosporidium parvum and T. gondii.
The data presented support the previously published hypothesis that the immune response to primary infection is dominated by MHC class II-restricted CD4+, IFN-γ-producing αβ T cells; for example, IFN-γ−/− and I-A−/− mice are as susceptible as TCRβ−/− mice. In any studies of knockout mice, important physiological mediators can fail to score as essential because of functional redundancy. This may be the case for class II MHC and IFN-γ in the secondary response. Nonetheless, neither of these components can be substituted for in the primary response. Thus, one can conclude that the primary infection is most likely characterized by a dominant Th1-type response, while the secondary response can be both primed and executed in the absence of class II MHC or IFN-γ.
Evidence that cells other than MHC class II-restricted T cells are activated during the primary response is provided in this study by data that the primary response is also sensitive to β2
m deficiency and influenced, albeit weakly, by IL-6 deficiency (Table ). This seems to parallel other instances in which more than one class of antigen-presenting molecule is important for the establishment of the full T-cell response (57
). Future studies will determine whether the cells sensitive to β2
m deficiency recognize antigens presented by classical class Ia MHC, but in a TAP-independent fashion, or TAP1-independent, class I-related molecules, of which there are a growing number of candidates, e.g., CD1, TL, RAE, H60, and any putative murine homolog of MICA/B that is expressed on activated human enterocytes (9
There is precedent for the involvement of T cells reactive to such antigens in the response to protozoal infection. Thus, murine T-cell types including TCRαβ CD4+
NK1.1 cells have been shown to be CD1 restricted (23
), and antibody synthesis supported by such cells was reported following infection by Plasmodium
, another apicomplexan (44
). One of the characteristics of such cells is an oligoclonal TCR repertoire, most utilizing Vα14Jα281 and a limited set of Vβs. Intriguingly, one such set of cells expressing Vβ3 has been documented exclusively in the intestine (25
). Although CD1-restricted, CD4+
cells are notable for their capacity to produce IL-4 within 30 min of anti-CD3 treatment (58
), they can also produce IL-2 and IFN-γ (3
). Hence the sensitivity of the antieimerian response to β2
m deficiency but not to IL-4 deficiency does not preclude a role for TCRαβ CD4+
NK1.1 cells. Interestingly, β2
m deficiency has been reported to reduce the number of IFN-γ-producing intestinal intraepithelial lymphocytes in response to intestinal L. monocytogenes
The rapidity and effectiveness of the secondary response to Eimeria
are both extremely high and represent hoped-for standards for oral vaccines for a plethora of infectious diseases. Moreover, antieimerian immunity is long-lived, in contrast to the commonly cited transient status of memory for mucosal pathogens (reviewed in reference 2
). Nonetheless, it had previously been inferred from antibody depletion studies that the requirements for the secondary response might be less than those for the primary response. For example, both anti-CD4 and anti-IFN-γ antibodies administered in vivo inhibited the primary but not the secondary response (38
). Heretofore, the interpretation of such studies had to be qualified by the uncertain capacity of administered antibodies to penetrate the GALT, where memory cells might reside. Such qualifications are offset by the current experiments that show that a memory response, either 100% complete or 95 to 99% complete, clearly develops in IFN-γ−/−
and MHC class II−/−
mice, respectively, even though these mice cannot develop primary responses of any magnitude. These data are quite different from those on the effects of αβ-T-cell deficiency that obliterates both the primary and secondary responses equally (Fig. ) (33
). Moreover, we show that the increased resistance of I-A−/−
mice is reflected in genuine transferable immunity (Table ). Such memory is not provided by γδ T cells or B cells (A. L. Smith and A. C. Hayday, submitted and unpublished studies), and it will be interesting to determine the nature of the cells that mediate such immunity.
The complete dependence of the memory response on αβ T cells (Fig. ) means that in the β2
mice and IA−/−
mice the memory response is most likely driven by different T-cell populations: those selected on β2
m-associated MHC in IA−/−
mice or those selected on class II MHC in β2
mice. One might hypothesize that a variety of different memory cells can be primed during the initial encounter with an antigen. Possibly these cells compete with each other for long-term survival and reactivation. This would be consistent with the demonstration that B cells compete with each other for a limited number of available niches during recirculation (11
). It would also seem consistent with the concept that Th1 and Th2 dichotomies reflect population effects rather than the exclusive and precise commitment of every cell clone to one particular phenotype (21
). Hence, in the absence of one population (e.g., IFN-γ-producing, MHC class II-resticted CD4+
αβ T cells) other antigen-experienced cell types may develop more successfully and subsequently act to provide memory. The factors that prevent similar functional redundancy in the primary response need to be clarified, but, until proven otherwise, it may simply be the time constraint in making a response de novo to a rapidly proliferating pathogen. This could likewise explain the capacity to “see” fully effective recall responses in mice, such as the IFN-γ−/−
strain, in which there was no measurable primary response.
The capacity to elicit pathogen-limiting memory responses in the absence of all molecules except for TCRαβ may be germane to vaccine programs that would be designed to recapitulate the efficacy of the anti-Eimeria response. Different adjuvants and vaccine delivery regimens are known to promote some effector functions better than others. The findings presented here suggest, perhaps surprisingly, that there may be room for some latitude in vaccine design. So long as appropriate, neutralizing epitopes can be presented and recognized, effective immunoprotective T cells may develop in multiple compartments.