Lacrimal inflammation (dacryoadenitis) : The nonobese diabetic (NOD) mouse model shows a lymphocytic infiltration of predominantly CD4
+ Th1 cells in the lacrimal gland as well as other organs, including the pancreas, submandibular, and thyroid glands. Male NOD mice show significant inflammatory lesions of the lacrimal gland from the age of 8 weeks, whereas female NOD mice do not show any changes until 30 weeks of age.
5 Although the incidence of diabetes and sialoadenitis in NOD females is higher than that in males, Takahashi et al.
5 have reported a significantly higher incidence of dacryoadenitis in males at all ages. Moreover, they have shown that testosterone increases the incidence of autoimmune lesions. These findings are in sharp contrast to clinical Sjögren’s syndrome, which is far more prevalent in females than in males and has been associated with androgen insufficiency. In addition, although significant attention has focused on lacrimal lesions in NOD mice, the only published study in the literature demonstrating altered tear secretion in these mice reported a reduction by 33% to 36% compared with wild-type animals,
6 a reduction rate that is modest compared with the profound decreased lacrimal function in Sjögren’s patients. The consequences of the reduced tear production on the ocular surface epithelium of these mice have also never been definitively demonstrated. Overall, the appearance of autoimmune diabetes before autoimmune exocrinopathy in the NOD mouse suggests that it is an excellent model of secondary, but not primary, Sjögren’s syndrome.
However, the immunopathogenic mechanisms involved in the NOD mouse are complex. For example, Robinson et al.
7 have demonstrated that NOD.B10.H2
b mice, in which the I-A
g7 segment of the MHC region has been replaced by the H2
b haplotype of C57Bl/6 mice, do not have autoimmune diabetes, but these congenic animals retain the characteristic feature of lymphocytic infiltration and dysfunction of the lacrimal gland. Therefore, it has been postulated that the appearance of T cells in the exocrine tissue, despite the lack of a corresponding insulitis, supports the concept that NOD mice have two independent autoimmune mechanisms, potentially rendering the NOD.B10.H2
b mouse a better model for primary Sjögren’s syndrome.
The MRL/MpJ-
fas +/
fas + (MRL/+) and MRL/MpJ-
fas lpr /
fas lpr (MRL/lpr) mouse models of Sjögren’s syndrome exhibit lacrimal gland infiltrates characterized by a predominance of CD4
+ T cells.
3 In contrast to the NOD model, the extent of the lacrimal gland inflammation is significantly greater in lacrimal glands of female MRL/+ and MRL/lpr mice than is observed in males,
8 resembling the difference observed in human Sjögren’s syndrome. Of note, in the MRL/lpr mouse the lacrimal gland inflammation has an earlier onset (1 month of age) and a greater severity at same ages as in the MRL/+ mice (onset at 3 months), indicating that the
lpr (lymphoproliferation) mutation accelerates, rather than causes the disease.
3 The defective lymphocyte apoptosis (thought to regulate T cells reactivity naturally) in MRL/lpr mice due to an
lpr mutation in a single autosomal recessive gene controlling the Fas antigen occurs systemically and outside the lacrimal gland, and therefore is not itself due to the microenvironment of the lacrimal gland in this mouse model.
9 According to Toda et al.
8 the
lpr gene itself does not cause severe immunopathologic lesions in the lacrimal tissue, since male C3H/lpr and
gld (generalized lymphoproliferative disease) mice show almost no lymphocyte accumulation in the lacrimal gland. These findings seem to support the hypothesis that Fas antigen and Fas ligand are not critical gender- and strain-independent determinants of autoimmunity in lacrimal tissue. The immunopathology of this model is unique, in that the predominance of IL-4 and B7-2 within the lacrimal gland lesions of MRL/lpr mice
10 suggest a largely Th2-type response, distinct from that in the NOD model. Overall, these data suggest the existence of potentially divergent immunopathogenic mechanisms to lacrimal autoimmunity in the mouse.
Autoimmune phenomena in F1 hybrids of New Zealand Black and New Zealand White (NZB/NZW F1) mice are comparable to MRL/lpr mice and are similarly more severe in females than in males,
8 but lesions in the lacrimal glands of the F1 mice become evident after only 4 months instead of 1 month.
3 The effect of lacrimal inflammation on the ocular surface of these mice is poorly described. In fact, Gilbard et al.
11 demonstrated the
lack of corneal epithelium abnormalities in NZB/NZW F1 mice along with a normal tear film osmolarity, despite the lacrimal gland infiltration, emphasizing the important point that lacrimal and ocular surface disease do not necessarily correlate.
There are several other murine models that are also associated with a predominant CD4
+ T lymphocyte infiltration of the lacrimal gland. The TGF-β1 knockout mouse, for example, has shown significant inflammatory cell infiltrates in the lacrimal gland between the ages of 2 and 4 weeks, whereas the globe itself exhibits a normal structure and phenotype on histologic examination.
12 Unfortunately, shortly after weaning, the mice begin to show the symptoms of a wasting syndrome and die between 3 and 4 weeks. Providing liquid diet supplements at the time of weaning prolongs survival of TGF-β1 knockout mouse almost twice as long as providing standard hard chow, thus enabling longer observation of these mice.
13 The shortened life of the TGF-β1 knockout mouse generally complicates their use as a model, particularly for studies involving testing of therapeutics.
Mice homozygous for an autosomal recessive mutation, alymphoplasia (
aly), which causes a systemic absence of lymph nodes and Peyer’s patches, have also been demonstrated to have infiltration of CD4
+ T cells into their lacrimal and salivary glands, lung, liver, kidney, and pancreas,
14 potentially serving as a model of Sjögren’s syndrome. In particular, lymphocytic infiltrates can be detected in the lacrimal gland from the age of 14 weeks in both males and females. Several factors complicate the use of these mice for study of ocular surface disease. First, no definitive data have been produced relating the lacrimal gland infiltration to altered tear secretion in these mice. Second, the absence of lymph nodes, together with a variety of other serious immune defects, including depressed baseline immunoglobulin production and isotype-switching, defective T cell function, and faulty homing responses, all confound the study of the effect of lacrimal insufficiency on the eye, as each of these factors alone or in combination may affect the cornea and ocular surface.
Finally, the IQI/Jic has recently been established as a new mouse model for primary Sjögren’s syndrome. Unlike the NOD model, in the IQI/Jic mouse the lymphocytic infiltration is well restricted to salivary and lacrimal glands. Konno et al.
15 have suggested that in this model, the early phase of adenitis is mediated by dendritic cells that promote induction of Th1-type immunity, which may also be potentiated by lacrimal epithelial cells that also function as secondary antigen-presenting cells (APCs). The general phenotype of this mouse model of Sjögren’s syndrome makes it quite attractive for the study of the early phases of the disease, even if the development of complete dacryoadenitis at an advanced age (9 months) makes it economically unattractive. Another attractive model for inducing salivary and lacrimal gland specific lymphocytic infiltration, but at a more rapid pace, has been developed by injecting mononuclear cells isolated from the inflamed submandibular salivary tissues of MRL/lpr mice intraperitoneally into
Scid mice. At 8 weeks after the injection, these
Scid mice exhibit inflammatory lesions in the salivary and lacrimal glands, but not in other organs, confirming the tissue-specificity of disease in these animals. Of interest, the injection of lymphocytes from the spleen of the MRL/lpr mice have not produced any inflammatory lesions in
Scid mice.
16
Autoantibodies : Human Sjögren’s syndrome is characterized by the presence of serum autoantibodies to the ribonucleoprotein particles SS-A (anti-SS-A/Ro, 52 kDa; anti-SS-A/Ro, 60 kDa) and SS-B (anti-SS-B/La),
17 and, as recently demonstrated, anti-120-kDa α-fodrin,
18 and anti-M3 and -M1 muscarinic acetylcholine receptors.
19 Autoantibodies are also present in the serum of NOD and MRL/lpr mice, but with a different pattern of expression
(Fig. 1) than what is observed in humans. The role of autoantibodies in NOD mouse has been postulated based on the observation that NOD Igμ
null mice (lacking B lymphocytes) maintain normal secretory function, even in the presence of focal infiltrates in the salivary gland, suggesting that humoral immunity plays a critical role in the pathogenesis of autoimmunity in these animals.
20 Furthermore, the infusion of anti-M3 muscarinic receptor antibodies into NOD-
Scid mice produces a decreased salivary response.
21 To date, there have been no published data about the influence of anti-M3 muscarinic receptor antibodies on tear production.
Finally, NFS/sld mice thymectomized at 3 days of age (3d-Tx NFS/sld) have also been used as a model of primary Sjögren’s syndrome to identify the autoantigen 120-kDa α-fodrin.
18 In particular, this autoantigen, purified from the salivary glands of 3d-Tx NFS/sld mice, postulated as the presenting antigen in salivary gland disease in Sjögren’s syndrome, has been demonstrated to be the product of caspase 3 cleavage of the 240-kDa α-fodrin, rendering this model ideal for study of the mechanism of apoptosis in Sjögren’s syndrome. However, the profound effects of the thymectomy on the endocrine-immune system limits the use of this model for long-term prospective studies.