Received: 30 June 2023 | Accepted: 11 July 2023 DOI: 10.1111/eve.13866
C L INI C A L COMMENTA RY
Immunohistochemistry in equine pathology: A brief overview Guido Rocchigiani
| Lorenzo Ressel Department of Veterinary Anatomy, Physiology and Pathology , University of Liverpool , Neston , UK
Correspondence : Guido Rocchigiani Email:
guido.rocchigiani@
liverpool.ac.uk
INTRODUCTION
Immunohistochemistry (IHC) is a technique which revolutionised the world of pathology when introduced in 1941 by Coons et al. ( 1941 ). One of the first immunohistochemical studies in horse was pub- lished in 1966 (Nairn & De Boer, 1966 ). Since then, there are 3965 PubMed- indexed articles at the time of writing this commentary. IHC uses primary antibodies to highlight any type of antigen ex- pressed by normal host cells, viruses, bacteria and tumoural cells, and such versatility made IHC an invaluable tool for research focus- sing at the same time on protein expression and morphology. While IHC more deep technical aspects are beyond the scope of this man- uscript and will therefore not be discussed (for additional data, see Ramos- Vara & Miller, 2014 ), we will briefly discuss the importance of IHC in equine diagnostic pathology, condensed in a few paragraphs. All the IHC markers mentioned in the present study refer to IHC performed on formalin- fixed paraffin- embedded (FFPE) tissue, as this represents the most diffuse IHC used in veterinary pathology.
INFECTIOUS AGENTS
As in humans and other animal species, IHC can be used to highlight and locate pathogens in tissue sections. A broad number of viruses, bacteria, fungi and parasites are detectable via IHC and in this sec- tion, the most relevant will be discussed. Equine herpesviruses (EHVs) are common pathogens able to in-
duce numerous clinical syndromes and lesions in the horse. IHC tar- geting EHVs is a useful diagnostic method, but such a technique is particularly important for equine herpesviral encephalomyelopathy (EHM). This condition, usually induced by EHV- 1, is characterised by viral replication within the CNS endothelial cells leading to thrombotic vasculitis and subsequent neural damage. While molecular tests (e.g. PCR) are able to identify the EHVs in any fluid and tissue, IHC (to- gether with in situ hybridisation) has the advantage to confirm clinical suspect of equine herpesviral encephalomyelopathy, when viral anti- gens are highlighted in endothelial cells (Pusterla et al., 2022 ).
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Bovine papillomavirus (BPV) types 1, 2 and 13 are considered the aetiological agents of equine sarcoids, the most common equine cu- taneous tumour. Light microscopical features of equine sarcoids are characteristically enough to make a diagnosis; nevertheless, IHC tar- geting multiple viral proteins has been performed for research pur- poses and a better understanding of the biology of these tumours. According to the literature, papillomaviral proteins E5 and E7 were observed in both epidermal and dermal neoplastic cells while L1 label- ling was observed only on the most superficial keratinocytes (Brandt et al., 2011 ). Equine coronavirus is a recently described pathogen associ-
ated with enterocolitis in horses (Zappulli et al., 2020 ). Among the diagnostic
tools used, immunohistochemistry is rarely reported,
with only a single study highlighting pathological features (Giannitti et al., 2015 ). In such paper, IHC was carried using as a primary anti- body targeting bovine coronavirus, due to their antigenic similarity. Bacteria can also be highlighted via IHC. While bacterial culture
and isolation from equine tissue is a solid principle of equine medi- cine, IHC can come in handy highlighting specific bacterial antigens for specific agents, such as Leptospira spp. (Szeredi & Haake, 2006 ) in the context of the tissue lesions. Similar concept applies to parasites: nonetheless, for most par-
asites, IHC is seldom used for diagnosis in the equine species with few records for Sarcocystis neurona (Henker et al., 2020 ), Neospora caninum (Anderson et al., 2019 ) and Leishmania spp. (Ortega- García et al., 2021 ). This is likely due to the reduced sensibility of such tech- niques compared to serology or molecular assays (e.g. PCR) (Guido Rocchigiani, personal observation).
TUMOURS AND OTHER ENDOGENOUS ANTIGENS
Immunohistochemistry, as highlighted by the manuscript in this issue by Hoerdemann et al. ( 2024 ), is an invaluable tool to reach a final di- agnosis in the majority of tumoural and proliferative lesions: identify- ing structural and/or inducible protein expressed by neoplastic cells
aaep.org/wiley-redirect Equine Vet Educ. 2024;36:12–15.
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