The formal verification of Artifact-centric (AC) systems is a subject of growing interest in the Service Oriented Computing (SOC) community, which can benefit from techniques developed for Multi-agent systems and knowledge reasoning and representation. In the present contribution we consider the verification of AC systems that do not necessarily satisfy boundedness and uniformity, the typical assumptions used to prove decidability of the model checking problem in this setting. We provide a partial model checking procedure for agent-based AC systems against a first-order temporal logic that includes modal operators for agent knowledge. Interestingly, we obtain this result by introducing a counterpart semantics for first-order modal logic, and by defining notions of simulation and abstraction for this setting. This allows us to generate finite abstractions of infinite-state AC systems, even when these are not bounded nor uniform, thus enabling us to perform verification also in cases not covered by the current state-of-the-art.