Objective—To investigate the direct interaction
between canine keratinocytes and live Malassezia
pachydermatis and thereby determine the role of these
organisms in the pathogenesis of epidermal hyperplasia
associated with Malassezia dermatitis in dogs.
Sample population—Primary canine keratinocyte
cultures established from skin samples obtained from
clinically normal dogs.
Procedure—The proliferative response of keratinocytes
co-cultured with Malassezia organisms for
1, 2, or 3 days was assessed by use of direct manual
counting (to determine the number of keratinocytes
in both the monolayer and the medium) and immunohistochemical
staining techniques involving antibodies
against proliferating cell nuclear antigen (PCNA)
and another cellular proliferation marker, Ki-67. The
potential cytotoxic effect of Malassezia organisms
was investigated by use of an apoptosis detection kit
to label keratinocytes co-cultured with M pachydermatis
that underwent apoptosis.
Results—No stimulatory effect of Malassezia organisms
on canine keratinocyte proliferation was detected
via cell counting and immunohistochemical techniques.
However, there was a significant increase in
dead keratinocytes in the medium with increasing
numbers of Malassezia organisms in the co-culture.
More apoptotic cells were observed in keratinocyte
monolayers co-cultured with high numbers of
M pachydermatis than there were in monolayers cultured
without Malassezia organisms, and the number
increased after prolonged incubation.
Conclusions and Clinical Relevance—M pachydermatis did
not stimulate canine keratinocyte proliferation in
vitro. The results suggested that the epidermal hyperplasia
observed in dogs with Malassezia dermatitis is
unlikely to be caused by a direct effect of the organism
on the keratinocyte cell cycle, but is likely to involve
other mechanisms. Am J Vet Res (2004;65:787–796)