METHODS
This was a prospective, in vivo, placebo controlled, randomized animal study. The University’s Institutional Animal Care and Use Committee approved all procedures prior to initiation. Twenty-four male Sprague-Dawley rats were randomized to 3 treatment groups (n = 8): monocytes; granulocytes, and; plasma, as a negative control.
Allogenic blood from 6 additional rats was collected into K2EDTA tubes via cardiac puncture. Allogenic, as opposed to autogenic, blood is commonly used in rat models because of low immunogenic response to blood from rats of the same strain and litter.29,30 The blood was then pooled and the red cells lysed by incubation with Red Blood Cell Lysis Buffer (Roche). The samples were then sorted into fractions containing monocytes and granulocytes using fluorescence activated cell sorting (FACS) using a FACSAria (BD Biosciences). Cells were sorted using Purified PE Mouse Anti-Rat CD11b/c antibodies (BD Pharmingen) specific to monocytes, granulocytes, macrophages, dendritic cells, and microglia, APC-Cy7 Mouse Anti-Rat CD45 antibodies (BD Pharmingen) specific to all hematopoietic cells except erythrocytes, and FITC Mouse Anti-Rat CD42d antibodies (BD Pharmingen) specific to megakaryocytes and platelets. 20 μL of 0.2 mg/mL CD11b/c, 20 μL of 0.2 mg/mL CD 45, and 10 μL of 0.5 mg/mL CD42d antibodies were added to 1 mL of condensed non-red cells collected from the 6 rats and incubated at room temperature in the dark for 15 minutes. A fraction containing only platelet-poor plasma was also collected. For all treatments the injection volume was 75 μL. Rats in the monocyte group were injected with 200,000 cells in platelet-poor plasma, those in the granulocyte group were injected with 900,000 cells in platelet-poor plasma, and rats in the plasma control group received only platelet-poor plasma. The cell concentrations were based on previous studies that documented these concentrations that are found in typical leukocyte-rich PRP preparations.13
The animals were anesthetized with isoflurane gas and then injected aseptically once into their right Achilles tendon. The left Achilles tendon was used as an un-injected control, giving a total of 48 total Achilles tendons studied. At days 7 and 14 post-injection, 4 rats from each group were euthanized and the Achilles tendons were harvested.
The tendons were fixed in neutral buffered formalin for 24 hours and then embedded in paraffin and sectioned sagittally at 12 μm. The tendons were then stained with hematoxylin and eosin (H&E) using standard histological protocols and examined by 3 individuals trained to assess cellularity and morphology. All samples were assigned unrecognizable numbers and randomized prior to examination by individuals. Cell counts were based on the number of nuclei present in 3 mid-tendon high-power fields (400x) per sample. Morphology was graded on a scale of 1 to 3, with 1 being a normal tendon and 3 having severe pathology with total loss of alignment and crimping on 3 low-power fields (100x) per sample (Figures 1A-1G).
Vascularity was assessed by immunohistochemical staining using Rabbit Polyclonal Anti-CD31 antibodies (Abcam), a marker for vascular endothelial cells, using a Vectastain ABC Kit (Vector Laboratories) system and the ImmPACT AEC Peroxidase (HRP) Substrate (Vector Laboratories). Following staining, automated image analysis was performed (Bioquant). Briefly, all areas that did not contain tendon were masked. CD31 positive areas were then quantified using global thresholding. Vascularity was then calculated as ratio of CD31 positive area to total tendon area. Analyses were performed on 3 mid-tendon medium-power (200x) fields per sample.
For cellularity and morphology, the results for the injected tendons were normalized to those of their contralateral untreated controls and reported as a percentage. Results for vascularity were compared directly between treated tendons. Differences were assessed between groups at each time-point using Independent Samples Median Tests. When significant differences were identified, pairwise comparisons were performed to identify the source of the differences. All analyses were conducted using SPSS (V22, SAS Institute) with significant differences determined for values of P < 0.05.
RESULTS
No significant differences in cellularity between groups were seen at day 7 (P = 0.368) (Figures 1A-1G). However, a significant difference in cellularity between groups was seen at day 14 (P = 0.014). Pairwise tests showed there to be a significant increase in the number of cells in the tendons treated with granulocytes from 221% and 249% in cellularity (P = 0.014) on day 14, as compared to both monocytes and plasma, respectively. Morphologically, no significant differences were seen between groups at either time-point (P = 0.091 for day 7 and P = 1.000 for day 14) (Figures 2A-2G). However, a significant improvement in morphology was observed from day 7 to day 14 in the granulocyte group from 60% to 165% (P = 0.029). Finally, no differences were seen in vascularity between treatment groups at either time-point (P = 0.368 for day 7 and P = 0.535 for day 14) (Figures 3A-3G).
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