Virus or not Virus?

Hi There
I have a sample that presents syncytia when inoculated on Vero, it's however impossible to observe syncytia when i subculture from Vero.
I performed the following experiment to determine if it's a chemical or viral induced effect:
1. sample was centrifuged to remove toxic cell debris, and splited into 3 fractions.
2. fraction-1 inoculated as is on vero to serve as control
3. fraction-2 is filtered through 0.22 micron and inoculated to vero
4. fraction-3 is filtered through 0.1 micron and inoculated to vero

results:
fraction-1 and fraction-2 presented syncytia (fewer syncytia for fraction-2)
fraction-3 (0.1 micron filtered) DID NOT present syncytia

Question:
Could someone help me describe all the possibilities as of the cause of the syncytia observe for the non filtered and the 0.22 micron filtered sample.
any thought is welcome as well suggestions for further experiment.
Thanks a lot