Sjogren's syndrome is an autoimmune disorder characterized by lymphocytic infiltrates and destruction of the salivary and lacrimal glands, and systemic production of autoantibodies to the ribonucleoprotein (RNP) particles SS-A (also called Ro RNA particle) and SS-B (also called La snRNP) (1, 2). However, the pathogenesis of this syndrome remains unclear. We established an animal model of primary SS in NFS/sld mutant mice thymectomized 3 days after birth (3d-Tx) (3). The T cell receptor V [Beta] 8 gene is preferentially expressed in these inflammatory lesions from the onset of disease, and high concentrations of salivary duct autoantibodies of immunoglobulin G (IgG) type were detected in sera from mice with autoimmune lesions (3). The 3d-Tx NFS/sld mice did not develop any other autoimmune lesions. To identify an organspecific antigen, we used protein immunoblot analysis to detect a salivary gland autoantigen reactive with affinity-purified IgG from sera of 3d-Tx NFS/sld mice; no reactivity was detected in tissue homogenates from the lung, liver, heart, kidney, pancreas, spleen, or brain (Fig. 1A) (4). Moreover, no reactivity was detected in tissue homogenates from the salivary glands in other mouse strains such as BALB/c and C3H/He. We purified the salivary gland autoantigen from tissue homogenates by fast protein liquid chromatography (FPLC). The fraction containing autoantigen activity was purified further by ion exchange high-performance liquid chromatography (HPLC). Autoantigen activity was recovered in fractions 27 and 28 (Fig. 1B). Protein immunoblot analysis of these fractions revealed a major band of 120 kD (Fig. 1B, inset) (4).

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To identify the 120-kD salivary gland protein, we electroblotted the band onto an Immobilon membrane and directly sequenced it with an Applied Biosystems 477A Protein Sequencer. The bands had a single NH2-terminal sequence. This sequence was identical to the [NH.sub.2]-terminal sequence of human [Alpha]-fodrin (nonerythroid [Alpha]-spectrin) (5): RQKLEDSYRFQFFQRDAEEL (6).

Proliferative T cell responses of spleen cells from 3d-Tx NFS/sld mice to the identified 120-kD antigen developed spontaneously at 4 weeks of age, consistent with the onset of the autoimmune lesions in the salivary gland (Fig. 1C) (7). This response increased at 8 and 12 weeks of age, then declined by week 16. In contrast, no response was detected with control antigens (fraction 40, lysozyme, and a-amylase). The spontaneous development of a proliferative T cell response to the 120-kD antigen is consistent with endogenous priming. We tested the 120-kD antigen-reactive T cells for additional properties to confirm the specific T cell responses. Splenic T cells from 8-week-old 3d-Tx NFS/sld mice challenged with the 120-kD antigen produced interleukin-2 (IL-2) and interferon-[Gamma] (IFN-[Gamma]) (Fig. 1D) (8). Thus, a potentially pathogenic T helper type 1 [(T.sup.H1)] T cell population may be spontaneously primed to the 120-kD salivary gland autoantigen early in the development of autoimmune lesions in the salivary glands of this mouse model.

To examine the organ specificity of the 120-kD [Alpha]-fodrin, we immunoblotted various tissue homogenates from 3d-Tx NFS/sld mice with a commercially available monoclonal antibody to [Alpha]-fodrin. More 120-kD [Alpha]-fodrin was detected in the salivary gland homogenate than in homogenates from other organs (Fig. 2A) (4). A very faint 120-kD band in brain, heart, and lung tissue homogenates seems to be a degradation product of intact [Alpha]-fodrin, because autoimmune lesions were not detected in these organs. In nonthymectomized NFS/sld mice, a negligible amount of 120-kD [Alpha]-fodrin was detected in the salivary gland or in other organs (Fig. 2A). These data suggest that the salivary gland expression of 120-kD [Alpha]-fodrin may be an initial response in the development of autoimmune lesions in the salivary glands. Polyclonal rabbit