The role of infections in autoimmune disease.

Autoimmunity occurs when the immune system recognizes and attacks host tissue. In addition to genetic factors, environmental triggers (in particular viruses, bacteria and other infectious pathogens) are thought to play a major role in the development of autoimmune diseases. In this review, we (i) describe the ways in which an infectious agent can initiate or exacerbate autoimmunity; (ii) discuss the evidence linking certain infectious agents to autoimmune diseases in humans; and (iii) describe the animal models used to study the link between infection and autoimmunity.

A virus-induced molecular mimicry model of multiple sclerosis.

Multiple sclerosis1 (MS) is an immune-mediated autoimmune demyelinating disease in humans. The initiating event in MS is unknown, but epidemiological evidence suggests that virus infections may be important and one possible mechanism for induction of infection-induced autoimmune disease is molecular mimicry. To test the ability of a virus encoding a self myelin mimic epitope to induce an autoimmune response, we have developed a mouse model wherein the immunodominant myelin epitope PLP139-151, or mimics of this epitope, were inserted into a nonpathogenic variant of Theiler's murine encephalomyelitis virus (TMEV). SJL mice infected with TMEV containing PLP139-151 or a mimic of PLP139-151 expressed by the protease IV protein of Haemophilus influenzae, sharing only 6/13 amino acids with the native epitope, developed an early-onset demyelinating disease associated with activation of CD4+ T cells reactive with PLP139-151. We have used this molecular mimicry model to further address the requirements for mimic epitope processing and presentation during infection and the requirements for TCR recognition and MHC binding of mimic epitopes. We have also investigated whether molecular mimicry may require multiple infections, with either the mimic-encoding virus or an unrelated virus, to initiate autoimmune disease. Finally, we have asked whether a virus encoding a molecular mimic has to directly infect the target organ to induce autoimmune disease. Overall, this virus-induced molecular mimicry model has provided critical information regarding the mechanisms by which infection-induced molecular mimicry can induce autoimmune diseases.

Cyclophosphamide, doxorubicin, and paclitaxel enhance the antitumor immune response of granulocyte/macrophage-colony stimulating factor-secreting whole-cell vaccines in HER-2/neu tolerized mice.

Tumor-specific immune tolerance limits the effectiveness of cancer vaccines. In addition, tumor vaccines alone have a limited potential for the treatment of measurable tumor burdens. This highlights the importance of identifying more potent cancer vaccine strategies for clinical testing. We tested immune-modulating doses of chemotherapy in combination with a granulocyte/macrophage-colony stimulating factor (GM-CSF)-secreting, HER-2/neu (neu)-expressing whole-cell vaccine as a means to treat existing mammary tumors in antigen-specific tolerized neu transgenic mice. Earlier studies have shown that neu transgenic mice exhibit immune tolerance to the neu-expressing tumors similar to what is observed in patients with cancer. We found that cyclophosphamide, paclitaxel, and doxorubicin, when given in a defined sequence with a GM-CSF-secreting, neu-expressing whole-cell vaccine, enhanced the vaccine's potential to delay tumor growth in neu transgenic mice. In addition, we showed that these drugs mediate their effects by enhancing the efficacy of the vaccine rather than via a direct cytolytic effect on cancer cells. Furthermore, paclitaxel and cyclophosphamide appear to amplify the T helper 1 neu-specific T-cell response. These findings suggest that the combined treatment with immune-modulating doses of chemotherapy and the GM-CSF-secreting neu vaccine can overcome immune tolerance and induce an antigen-specific antitumor immune response. These data provide the immunological rationale for testing immune-modulating doses of chemotherapy in combination with tumor vaccines in patients with cancer.

The collaboration of both humoral and cellular HER-2/neu-targeted immune responses is required for the complete eradication of HER-2/neu-expressing tumors.

HER-2/neu (neu) transgenic mice (neu-N mice), which express the nontransforming rat proto-oncogene, demonstrate immunological tolerance to neu that is similar to what is encountered in patients with neu-expressing breast cancer. We have shown previously that a significant increase in neu-specific T cells, but no induction of neu-specific antibody, is seen after neu-specific vaccination in neu-N mice. In contrast, a significant induction of both neu-specific T-cell and antibody responses is found in nontoleragenic FVB/N mice after vaccination. These mice are fully protected from a s.c. challenge with NT cells, a mammary tumor cell line derived from a spontaneous tumor that arose in a neu-N mouse, whereas neu-N mice are not. In this study, we demonstrate that CD4+ T cell-depleted FVB/N mice show no induction of neu-specific IgG after vaccination and are unable to reject an NT challenge (0 of 10 mice were tumor free). Conversely, the depletion of natural killer cells has no effect on vaccine-mediated tumor rejection (100% of mice were tumor free). In CD8+ T cell-depleted animals, where vaccine-induced neu-specific IgG titers were normal, NT growth was delayed, but only 10% of mice remained tumor free, demonstrating that neu-specific IgG alone is insufficient for protection from NT challenge. To directly assess the necessity for the combination of neu-specific cellular and humoral immune responses, severe combined immunodeficient mice were given an adoptive transfer of CTLs plus IgG derived from FVB/N mice. Animals that were given CTLs that recognized an irrelevant antigen plus neu-specific IgG developed tumors at a rate similar to CD8+ T cell-depleted FVB/N mice. Animals receiving an adoptive transfer of neu-specific CTLs plus control IgG derived from naive FVB/N mice were only partially protected from NT challenge (50% of animals were tumor free). However, only animals receiving the combination of neu-specific CTLs and neu-specific IgG were fully protected from NT challenge (100% of animals were tumor free). These studies specifically define the immunological requirements for the eradication of neu-expressing tumors in this model system, demonstrating that both cellular and humoral neu-specific responses are necessary for protection from an NT challenge. These data suggest that vaccines optimized to induce maximal T- and B-cell immunity to neu, and possibly to similar putative tumor-rejection antigens, may lead to more potent in vivo antitumor immunity.

HER-2/neu is a tumor rejection target in tolerized HER-2/neu transgenic mice.

HER-2/neu (neu-N) transgenic mice, which express the nontransforming rat proto-oncogene, develop spontaneous focal mammary adenocarcinomas beginning at 5-6 months of age. The development and histology of these tumors bears a striking resemblance to what is seen in patients with breast cancer. We have characterized the immunological responses to HER-2/neu (neu) in this animal model. neu-positive tumor lines, which were derived from spontaneous tumors that formed in neu-N animals, are highly immunogenic in parental, FVB/N mice. In contrast, a 100-fold lower tumor challenge is sufficient for growth in 100% of transgenic animals. Despite significant tolerance to the transgene, neu-specific immune responses similar to those observed in breast cancer patients can be demonstrated in neu-N mice prior to vaccination. Both cellular and humoral neu-specific responses in transgenic mice can be boosted with neu-specific vaccination, although to a significantly lesser degree than what is observed in FVB/N mice, indicating that the T cells involved are less responsive than in the nontoleragenic parental strain. Using irradiated whole-cell and recombinant vaccinia virus vaccinations we are able to protect neu-N mice from a neu-expressing tumor challenge. T-cell depletion experiments demonstrated that the observed protection is T cell dependent. The vaccine-dependent neu-specific immune response is also sufficient to delay the onset of spontaneous tumor formation in these mice. These data suggest that, despite tolerance to neu in this transgenic model, it is possible to immunize neu-specific T cells to achieve neu-specific tumor rejection in vivo. These transgenic mice provide a spontaneous tumor model for identifying vaccine approaches potent enough to overcome mechanisms of immune tolerance that are likely to exist in patients with cancer.