Immune mediated keratitis (IMMK) is primarily a non-ulcerative keratitis in horses causing intermittent ocular pain, eventually resulting in visual impairment. Affected horses typically respond to immunomodulatory treatment. However, the underlying cause of the disease remains enigmatic. The current study was undertaken to investigate the presence of autoantibodies in horses with immune mediated keratitis. Using 28 horses with IMMK and 27 healthy controls screening for serum autoantibodies against the corneal proteome using indirect immunofluorescence, one-dimensional (1DE) and two-dimensional electrophoresis (2DE) with subsequent western blot analysis was performed followed by mass spectrometric identification of bands or spots of interest. Indirect immunofluorescence did not reveal a difference in immune response towards corneal proteins between healthy horses and those with IMMK. Using western blot analysis some horses affected by IMMK (4/28) showed a single band (1D) or a single spot (2DE) (5/28) not detected in healthy controls. The corresponding spot was identified as maspin (SERPINB5), a protein responsible for the inhibition of corneal vascularisation, cell migration and cell adhesion to the extracellular matrix. Tests with a recombinant human protein commercially available did not verify blot findings, but the human protein may not be fully cross-reactive. Still, maspin might play a role in some cases of equine IMMK. Further research is needed to clarify the etiology of this disease.