Use of in situ hybridisation (ISH) to detect mRNA encoding immunoglobulin light and/or heavy chains has the advantage that its targets are confined to immunoglobulin-producing cells. Oligonucleotide and peptide nucleic acid cocktails have been available for some years in kit form for manual bench-top use with methodology very similar to that used for IHC. For ease of assessment and permanence of stained sections, these have always been available with chromogenic, rather than fluorescent, substrates. With the rapid expansion of automated IHC, it is perhaps surprising that automation for ISH has lagged behind, but this probably reflects a relatively low volume of ISH requests compared with throughput of IHC by most modern histopathology laboratories. Also, particularly with decalcified bone marrow trephine specimens (looking for light chain restriction in suspected plasma cell neoplasia), a moderately high rate of uninformative tests has, in some centres, dampened enthusiasm for developing ISH further. However, the performance of ISH probe cocktails using automated methods is more consistent and more informative. The accompanying article by Elderfield reports their initial, very encouraging, experience of adopting this approach at the Hammersmith Hospital. Their laboratory is one of relatively few in the UK where automated ISH is used for kappa and lambda assessment in suspected plasma cell neoplasia; experience elsewhere is equally positive.
One major limitation of the ISH approach in haematopathology remains; its relative lack of sensitivity.
In a small number of specialised histopathology laboratories, IHC is used successfully to demonstrate kappa and lambda expression by lymphoid cells other than plasma cells. However, for most laboratories, high background staining relative to any specific staining is a confounding factor, particularly as levels of lymphoid cell surface expression of immunoglobulin molecules are relatively low. It would therefore be immensely valuable to use ISH as an alternative, avoiding any need to assess the specific signal against background noise. However, apart from plasma cells, most normal and neoplastic B lymphoid cells produce light chain mRNAs in only small amounts, in keeping with their small requirement for producing the final proteins. With current probe cocktails, this is generally below the threshold of detection even with the adoption of automation and its greater consistency of performance.
It will be exciting to see the biotechnology industry rise to this challenge in the next few years, as automated ISH finds wider applications in histopathology practice.