Insulin-dependent diabetes mellitus (IDDM) develops after an individual's insulin-producing β cells in the pancreatic islets of Langerhans are destroyed by reactive T lymphocytes. This process is multifactorial, involving host genes, autoimmune responses, cytokines, and environmental factors (
2,
8,
18). The evidence for environmental influence is several pronged. First, studies of monozygotic twins in which one has diabetes but the other does not show a discordance rate of approximately 30 to 50% (
18,
25). Second, more than 80% of cases of IDDM occur in children with no family history of diabetes (
18,
25). This evidence is reinforced by linking the aberrant immune responses of several autoimmune diseases, including IDDM, with somatic (antigen driven) rather than germline mutation (
27,
40) and by analyzing epidemiologic surveys that associate multiple virus infections with IDDM (
2,
9,
10,
30,
31).
For example, fulfilling Koch's postulates, coxsackievirus, which has been linked to diabetes (
10,
31), was isolated from the pancreas of a human with acute-onset diabetes and, upon transfer, induced IDDM in an animal model (
42). Several systemic viral infections in humans preceded destruction of islets of Langerhans accompanying mononuclear cell infiltration (
12). In addition, 12 to 20% of children infected congenitally with rubella have IDDM (
9,
19,
30). Finally, in several model systems, viruses directly or indirectly cause IDDM (
9,
11,
19,
20,
22,
23,
30). However, despite this compelling evidence, in the vast majority of cases, no infectious agent (virus) has been uniformly identified.
This paper directly addresses the reasons for this dilemma. A transgenic mouse model is used in which a known viral gene (the nucleoprotein [NP]) of lymphocytic choriomeningitis virus (LCMV) is expressed in β cells (
22). No injury to these cells occurs throughout an animal's life unless it is later exposed to the same virus. The kinetics of IDDM onset and severity of disease are also dependent on expression of the viral transgene in the thymus as well as in β cells (
32), on the numbers and affinity of antiviral cells that escape negative selection and survive in the periphery (
13,
32,
34,
36), on the host's major histocompatibility complex (MHC) background (
32,
34), and on the expression of MHC molecules (
35,
37) as well as T-cell activation molecules (
36) in the islets' milieu. Although the events by which mononuclear cells are activated, infiltrate the islets, and destroy β cells, leading to hypoinsulinemia and hyperglycemia, are relatively clear in transgenic mice infected with the same virus, the role played by unrelated or other related viruses in causing IDDM is not.
This model allows us to address two fundamental issues. First, what is the number of effector cells required to cause disease? Second, what is the role played by unrelated or related viruses in causing IDDM? As expected, our results indicate that infections by vaccinia virus (VV) or Pichinde virus, representing viruses that do not generate cytotoxic T lymphocytes (CTL) cross-reactive with LCMV Armstrong (ARM) strain NP, the viral protein expressed on β cells, do not cause IDDM. Among the four strains of LCMV, a hierarchy of IDDM relatedness occurred: i.e., the LCMV strains E-350, Pasteur, and Traub elicited both CTL and antibody responses that cross-reacted with LCMV ARM and the LCMV ARM NP, but only ARM or E-350 infection elicited IDDM. The critical difference uncovered was that ARM and E-350 generated a higher CTL NP precursor (pCTL) frequency, of at least 1 or more CD8+ CTL per 1,000 splenic lymphocytes, which were specific for theH-2d (Ld )-restricted LCMV NP epitope. In contrast, Traub and Pasteur generated at least 8- to 20-fold fewer pCTL, respectively, that recognized the same LCMV ARM NP epitope. Furthermore, the molecular basis of why the Pasteur and Traub strains failed to generate sufficient levels of anti-LCMV NP (self) CTL is uncovered. The major implications of our finding for the identification of etiologic agent(s) that may cause an autoimmune disease like IDDM, the molecular basis by which cross-reactive viruses may or may not cause autoimmune disease, the quantification of numbers of antigen-specific cells required to cause IDDM, and the implications for successful immunotherapy are discussed.
DISCUSSION
The results reported here establish that the generation of CTL capable of cross-reacting with viral (“self”) antigens in pancreatic β cells is necessary but not sufficient to initiate IDDM. Disease followed only when the quantity of cross-reactive CTL exceeded a critical threshold, which, in this model, was 1/1,000 pCTL (or 1/50 to 1/100 LCMV-specific CD8
+ T cells). IDDM did not occur with 8- to 10-fold-fewer pCTL or LCMV-specific CD8
+ T cells. To understand the rules by which viruses can cause IDDM, we devised an in vivo murine model in which a viral gene was expressed in pancreatic β cells and passed on to progeny mice (i.e., the viral transgene became a self antigen). Previous experiments determined that expression of the transgene, per se, failed to initiate β-cell injury and resultant IDDM, unless either a specific cytokine like IFN-γ (
13,
37) or an activation molecule like B7.1 (
36) was coexpressed in the islet milieu in β cells or a systemic infection was initiated by the same virus from which the β-cell-expressing transgene originated (
22,
32). Our interest here was to determine if other viruses, closely or distantly related or unrelated could also cause IDDM.
First we analyzed four LCMV strains selected for their structural homology, CTL-generating capacity, and IDDM production. We found that only LCMV strain E-350, which shared 557 out of 558 aa with LCMV ARM NP (>99% homology) (Fig.
1) caused IDDM. Within the LCMV ARM NP used as the self antigen was the 10-aa NP aa 118 to 127 peptide that constituted the single immunodominant epitope recognized by
H-2d BALB mice. LCMV E-350 shared all 10 of these NP amino acids with LCMV ARM. Transgenic mice expressing the NP of LCMV ARM, when challenged with E-350, generated CTL that recognized NP aa 118 to 127. Furthermore, these E-350-specific CTL were generally equivalent in number to the CTL generated by LCMV ARM. After either infection, IDDM was evident from the characteristic hyperglycemia, hypoinsulinemia, and mononuclear cells infiltrating into the islets of Langerhans and participating in β-cell destruction (Fig.
3 and Table
2). Although sequence comparison between the Traub and ARM LCMV strains showed complete homology at the CTL epitope NP aa 118 to 127 (Fig.
1), and this LCMV ARM NP epitope was recognized by CTL generated in response to Traub infection (Table
1), Traub generated eightfold-fewer pCTL than ARM. Traub infection produced no IDDM, because this smaller number was not sufficient to cause the disease (Fig.
2 and
3). In agreement with our findings, a sevenfold reduction in pCTL frequency following DNA immunization for LCMV leads to vaccine failure (
28). Additionally, Traub differed from ARM at the flanking sequence NP aa 131 Thr→Ala. This suggested that the substitution in position NP aa 131 may alter antigen processing of the LCMV NP epitope. Experiments with intracellular cytoplasmic antigen processing showed that the single change at NP aa 131 diminished the presentation of antigen, although the amounts of transfected and expressed protein were equivalent for ARM and Traub. It is likely that the biologic consequence of this point mutation was the inability of Traub to cause IDDM. The importance of flanking sequences in antigen processing has been shown previously (
7,
16,
26); however, the model described here provides evidence for an in vivo biologic consequence.
Comparison of NP aa 118 to 127 (Fig.
1) of Pasteur with that of ARM showed 3 aa substitutions: the NP immunodominant epitope, aa 119 Pro→Leu, 120 Gln→Lys, and 121 Ala→Thr. Undoubtedly, these differences accounted for the 20-fold discrepancy in pCTL frequency between LCMV ARM and LCMV Pasteur and the inability of LCMV Pasteur infection to cause IDDM in transgenic mice expressing LCMV ARM NP (Fig.
2 and
3 and Table
2).
From the studies recorded here, two rules for the initiation of IDDM emerged. First, CTL must cross-react with a gene expressed in β cells, and, second, a sufficient number of CTL must be present. VV, a DNA virus with no structural relationship to LCMV ARM, failed to generate CTL that recognized the LCMV ARM NP transgene and, consequently, did not cause IDDM when inoculated into the transgenic mice (Fig.
2).
Pichinde virus is distantly related to LCMV ARM, since both viruses are members of the
Arenaviridaefamily, but Pichinde virus did not generate CTL that cross-reacted with LCMV ARM NP, despite inciting antibodies that recognized ARM NP. Pichinde virus infection did not produce IDDM (Fig.
2). Clone 13, an LCMV variant derived from the ARM strain (
1), shares complete homology with ARM NP (
29). However, clone 13, when inoculated (2 × 10
6 PFU i.v.) into mice, suppressed humoral and cellular immune responses because of its selected tropism for and associated destruction of interdigitating dendritic cells in the white pulp (
3). In comparison, ARM is tropic for F4/80-positive macrophages in the red pulp and does not cause injury of professional antigen-presenting cells (
3). In our experiments here, 7 days after clone 13 infection, no CTL were detected, although low levels (less than 1 per 50,000 pCTL by frequency analysis) appeared several weeks later, when progenitor cells from the bone marrow had repopulated the spleen and lymph nodes and differentiated into dendritic cells (M. B. A. Oldstone, A. Tishon, and P. Borrow, unpublished data). Additionally, inoculation of the transgenic or normal control mice with VV ARM NP failed to elicit a primary CTL response to LCMV ARM, although by days 45 to 60, we identified secondary (memory) CTL and pCTL frequencies of 1/15,500. Thus, although the Traub and Pasteur strains of LCMV generated primary CTL responses, the numbers of CTL generated (1/8,550 to 1/19,300 functional CTL) were insufficient to initiate significant β-cell destruction for development of IDDM. In contrast, the E-350 strain made, on average, 1/1,003 functional CTL able to cross-react with the LCMV ARM NP transgene in β cells and destroyed sufficient β cells to cause IDDM. Our findings indicated that the same generic virus, i.e., LCMV, causing infection in a genetically identical inbred mouse strain may (LCMV ARM or LCMV E-350) or may not (LCMV Traub or LCMV Pasteur) cause IDDM, depending on the strain or variant of virus involved. This suggests that one might note apparently similar viruses in two genetically identical people, i.e., monozygotic twins, but the viruses would cause IDDM in one and not the other because they are not the same.
Several other factors might have underlain the inhibition of IDDM seen here. First, the β-cell target could have had few or no MHC class I molecules and failed to present the autoantigen. Such a scenario has been reported in the RIP LCMV NP transgenic model when the IFN-γ gene was disrupted or when MHC molecules were retained in the endoplasmic reticulum due to the E3 transcriptional unit of adenovirus (
35,
37). This possibility is not likely in our current studies, since sufficient MHC and transgenic peptides were expressed on the β cells to allow recognition by CTL primed against LCMV ARM NP. A second possibility is that sufficient CTL were available, but their affinity was too low for activation and the resultant target cell lysis. This possibility is also unlikely, because the doses of peptide required for activation and lysis of CTL generated by Pasteur and Traub infections, which did not induce IDDM, were similar (10
−8 to 10
−9 M) to that for CTL from the LCMV ARM strain (10
−8 to 10
−9 M), which did cause IDDM. The third possibility, and the one that proved true, was the need for a sufficient threshold of effector CTL to cause IDDM.
The fact that a finite number of CTL rather than an all-or-nothing response is required to cause an autoimmune disease has important ramifications. Within a viral family like LCMV, some strains cause autoimmune disease and others do not. Yet, neither serologic markers nor cell proliferation assays necessarily distinguish the strains causing disease from those not causing disease. In the studies performed here, virus strains that caused or did not cause IDDM all generated antibodies and CTL that cross-reacted with the self (viral) antigen in the β cells responsible for IDDM. Furthermore, high homology (>96%) was noted between such viruses. Therefore, the interpretation of previous and current epidemiologic data utilizing serology or proliferation assays or molecular probes to detect regions shared among viral family members and assign a cause for IDDM or other autoimmune diseases is of questionable reliability.
Finally, to be successful in the treatment of autoimmune disease, reduction of the effector T cells rather than their elimination may be all that is necessary. Recently, using adoptive transfers and peptide blockers (
24,
33), we determined that IDDM did not occur when precursor frequency was lowered from 1/800 to 1/5,000 virus (self)-specific biologically active lytic CTL. We are currently quantitating the precise number of effector CTL required and evaluating several strategies to lower the number to a level that would circumvent autoimmune disease.