Enrichment of non-apoptotic human spermatozoa after cryopreservation by immunomagnetic cell sorting

Cryopreservation increases the rate of apoptotic spermatozoa with decreased capability to fertilise oocytes. In order to optimise the fertilisation rates, especially in assisted reproduction the use of apoptotic sperms should be avoided. Early events of apoptosis in cryopreserved spermatozoa are not detectable by conventional methods. However, the surface of apoptotic spermatozoa is characterised by externalisation of phosphatidylserine (PS), which has a high affinity to Annexin V. Therefore, colloid paramagnetic Annexin-V-conjugated microbeads (AN-MB) were tested for their ability to eliminate apoptotic spermatozoa from a total of 40 fresh and in TEST yolk buffer cryopreserved semen samples which were provided by 15 healthy volunteers. By passing through a magnetic field (MiniMACS, Miltenyi Biotec) the sperm suspensions were divided into 2 sperm fractions depending on bound magnetic Annexin V-microbeads (AN-MB) to spermatozoa. As additional markers of apoptosis CD95 (Fas, APO-1) on the sperm surface and activated caspases in the cytosol were detected in both fractions. Supplementary investigations comprised eosin-supravital staining and computer assisted sperm motion analysis. The separation was supervised by flow cytometric analysis of spermatozoa labelled with FITC-conjugated anti Annexin V-antibodies. Analyses of the magnetic inactive sperm fraction (AN-MB-negative) showed CD95 on 0.6 +/- 0.3% (X +/- SEM) of spermatozoa and only 3.2 +/- 0.5% were stainable with eosin, whereas, 40.6 +/- 6.7% of the remaining cells in the column appeared to be CD95 positive and 99.8 +/- 0.1% stainable with eosin after cryopreservation. Indeed the overall amount of CD95 positive spermatozoa did not significantly increase after cryopreservation (2.5 +/- 0.5% vs. 4.3 +/- 1.2%; p > 0.05). Activated caspases were found in 21.8 +/- 2.6% of the spermatozoa in fresh and in 47.7 +/- 5.8% of cryopreserved semen samples (p < 0.01). The separation procedure of the cryopreserved spermatozoa reduced significantly the quantity of those containing activated caspases to 9.3 +/- 2.2% within the AN-MB-negative fraction. In contrast 89.1 +/- 2.3% of AN-MB-positive sperms showed activation of these proteolytic enzymes. Flow cytometric analyses using FITC-conjugated anti Annexin V-antibodies for monitoring of AN-MB-binding to spermatozoa showed 5.2 +/- 1.0% labelled spermatozoa in the AN-MB negative fraction and 72.6 +/- 2.7% labelled spermatozoa in the AN-MB positive one. There was no significant influence of the separation column and the magnetic field on the sperm functions. The passage through the column led to a sperm loss of 0.8 +/- 1.2%. Conclusion: The binding of paramagnetic Annexin V-conjugated microbeads is an excellent method to eliminate spermatozoa at early apoptotic stages from cryopreserved semen samples. A deleterious influence of the separation column and the magnetic field on the spermatozoa was not observed.