  The realtime follow-up of neutrino events is a promising approach to search
for astrophysical neutrino sources. It has so far provided compelling evidence
for a neutrino point source: the flaring gamma-ray blazar TXS 0506+056 observed
in coincidence with the high-energy neutrino IceCube-170922A detected by
IceCube. The detection of very-high-energy gamma rays (VHE, $\mathrm{E} >
100\,\mathrm{GeV}$) from this source helped establish the coincidence and
constrained the modeling of the blazar emission at the time of the IceCube
event. The four major imaging atmospheric Cherenkov telescope arrays (IACTs) -
FACT, H.E.S.S., MAGIC, and VERITAS - operate an active follow-up program of
target-of-opportunity observations of neutrino alerts sent by IceCube. This
program has two main components. One are the observations of known gamma-ray
sources around which a cluster of candidate neutrino events has been identified
by IceCube (Gamma-ray Follow-Up, GFU). Second one is the follow-up of single
high-energy neutrino candidate events of potential astrophysical origin such as
IceCube-170922A. GFU has been recently upgraded by IceCube in collaboration
with the IACT groups. We present here recent results from the IACT follow-up
programs of IceCube neutrino alerts and a description of the upgraded IceCube
GFU system.
