Gene expression in the skin of tick-resistant and tick-susceptible cattle prior to and following infestation with Rhipicephalus microplus

The objective of this work was to examine the change in gene expression in the skin of tick-resistant and tick-susceptible cattle following infestation with the cattle tick, Rhipicephalus microplus. It was also considered important to note innate breed differences in gene expression prior to tick challenge. Skin biopsies (8 mm diameter and ~ 1 cm thickness) were taken from tick-resistant yearling Brahman females, tick-susceptible yearling Holstein-Friesian females, and from yearling Santa-Gertrudis females of both high and low resistance to ticks, prior to tick challenge and following tick challenge at sites where tick larvae were feeding. RNA extracted from biopsies was analysed using bovine Affymetrix whole genome short-oligonucleotide microarrays for global gene expression. Statistical analysis was performed using MAS5, RMA and GCRMA, and the final list of differentially expressed transcripts was restricted to those that were significant in all three analyses. Over 500 transcripts were found to be differentially expressed in the skin of tick-naïve animals compared with tick-infested animals, many of which are involved in cellular processes such as cellular adhesion and biological regulation, some were involved in the structural integrity of the dermis (such as collagen), and some were of unknown function. Further quantitative RT-PCR analysis was undertaken on skin samples from Santa-Gertrudis animals at 6 hours, 24 hours, 7 days and 21 days post infestation, using genes coding for cytokines, chemokine ligands and receptors, and several transcription factors, to outline the infiltration of cells to the tick attachment site in the first three weeks of infestation before stable tick-resistance is developed. Skin samples were also examined using histological and immunohistochemical techniques to confirm the types of cells infiltrating the dermis at tick attachment sites in the different breeds of cattle. Innate differences in gene expression, and cell types present in the skin, were noted between the breeds prior to tick infestation and following the stabilisation of resistance/susceptibility to the cattle tick