The polybasic furin cleavage site, though novel among lineage B sarbecoviruses, has precedent for arising naturally in coronaviruses (recombination/insertion, in-host evolution), and the predicted O-linked glycans flanking it imply the involvement of a functioning immune system in a live host, together arguing against generation by passage in cell culture.
Reasoning
Two sub-points. (1) Although no lineage B sarbecovirus has a polybasic cleavage site, insertions producing such sites do occur elsewhere in coronavirus evolution (other human betacoronaviruses such as HKU1 carry them), so the FCS can be gained by natural mechanisms - recombination or template-switching insertion in an animal host, or in-host selection during transmission. (2) The insertion is predicted to add O-linked glycans, which typically arise under immune selection (a ‘mucin-like domain’ shielding epitopes). Immune selection implies replication in an immunocompetent live host (animal or human), not in immortalised cell culture, so a cell-passage origin is disfavoured. Together these support natural acquisition of the FCS in vivo - in an animal host or during early human transmission - rather than laboratory cell-culture generation.
Validity verdict — approved (checked)
Reconstructed as two independent load-bearing steps, both narrowly targeting the cell-culture-passage scenario (not lab origin in general). Step (1): natural mechanisms (recombination/template-switch insertion, in-host selection) can produce a polybasic FCS, with precedent in other betacoronaviruses (HKU1) → so FCS presence does not require a lab. This is a straightforward existence/possibility step; granting the precedent premise, “FCS is naturally acquirable” follows. Step (2): predicted O-linked glycans → arise under immune selection → require an immunocompetent live host → not immortalised cell culture → cell-passage origin disfavoured. Conditional on the stated premise (“O-linked glycans imply the involvement of a functioning immune system in a live host”), the chain to “cell-culture generation disfavoured” is valid — cell lines lack the adaptive immune selection the premise posits. Probed undercutting defeaters: (a) glycans are only predicted, and glycosylation machinery exists in cell lines — but this attacks the premise that the predicted sites imply immune selection, which we grant here (its truth is priced at step 8); (b) the argument only rules out cell-culture passage, leaving engineering / in-vivo lab passage untouched — but the statement is already scoped to “cell-culture origin,” so this is not a validity defeat. No defeater survives that denies only the inference while granting the premises. Approved; the checked trace is the two-step reconstruction above.