Recombinant candidates reduced P. salmonis cell damage in SHK-1 by up to 47%

Executive summary

Safe alone, protective as infection progresses

ISR-BIO evaluated two recombinant candidates in SHK-1 cells under two conditions: protein-only exposure, which measures safety, and co-incubation with P. salmonis, which measures protection. Without infection, both candidates were safe (0–10% basal cytotoxicity). With infection, damage in the untreated control rose from 11% to 58% between days 6 and 10. Against that damage, Candidate A protected dose-dependently, reducing it by nearly half, while Candidate B reduced it by 23 percentage points at 0.25 and 0.5 µg/mL. The reading is direct: less damage than the infected control means protection against P. salmonis.

Modality

Safety and cellular-protection assessment of two recombinant candidates

Cell system

SHK-1

Conditions

Protein alone (safety) and co-incubation with P. salmonis (protection, days 6 and 10)

Primary metric

Cell damage (cytotoxicity %) versus same-day infected control

Configuration

0.25–2 µg/mL in protein-only exposure; 0.25–1 µg/mL in co-incubation

Key result

Up to −47% damage versus infected control (Candidate A, 1 µg/mL, day 10)

01 · Context

Reading cytotoxicity as protection

A protein-only readout answers a safety question: does the candidate damage cells in the absence of infection? Co-incubation answers the relevant question: when P. salmonis damages cells, does the candidate prevent it?

In each co-incubation panel, the dashed line is not a healthy-cell control: it is the untreated infected control, the damage caused by infection itself that day. Every bar below that line is damage the candidate prevented. This study uses three references: uninfected basal control (0%), infected control on day 6 (11%), and infected control on day 10 (58%).

02 · Challenge

Infection progresses between readouts

The infected control rises from 11% on day 6 to 58% on day 10: damage increases 5.3-fold as infection progresses. That defines how each panel should be read. On day 6, damage is still low, and matching the control means the candidate adds no damage during early infection. On day 10, with 58% damage established, protection becomes measurable.

Protection can only be quantified when there is damage to prevent. Each value is therefore compared with the infected control from its own day, without transferring readings between panels.

03 · Approach

Bars grouped by dose, candidate, and time

Protein-only incubation was assessed for six days at 0.25, 0.5, 1, 1.5, and 2 µg/mL against an uninfected basal control. Co-incubation with P. salmonis is shown at 0.25, 0.5, and 1 µg/mL on days 6 and 10, each against its untreated infected control.

Bars compare Candidates A and B at each dose, while the dashed line marks the control in each panel. The data were supplied without error bars; comparisons are described as observed values.

04 · Results

Protection emerged as infection progressed

With protein alone, both candidates were safe: Candidate A recorded 8%, 5%, 6%, 7%, and 6% between 0.25 and 2 µg/mL, and Candidate B recorded 5%, 4%, 1%, 0%, and 10%, versus a 0% basal control. Neither damaged cells on its own.

On day 6 of co-incubation, infected-control damage was still 11%, and both candidates stayed between 9% and 11%: they added no damage during the early phase of infection, when there was little damage to prevent.

On day 10, the infected control reached 58% damage and protection became visible. Candidate A reduced it dose-dependently: 58% at 0.25 µg/mL, 35% at 0.5, and 31% at 1 µg/mL—27 percentage points below control. Candidate B kept damage at 35% at 0.25 and 0.5 µg/mL—23 pp below control—and recorded 70% at the highest dose, a point that requires confirmation with replicates.

Observed cell-damage values; pp = percentage points versus the 58% infected control. No significance testing or bacterial-load measurement is reported here.

05 · What it means

A protective signal that grows with infection

The two recombinants meet the two conditions expected of a candidate: they do not damage cells on their own, and they reduce damage when infection is present. Being at control level on day 6 is not absence of effect: infection damage was only 11%, leaving little to protect. When damage rose to 58%, Candidate A reduced it by almost half with a dose-dependent profile, the pattern expected of real protection.

These in vitro results justify next steps: confirm protection with biological replicates and statistical analysis, measure bacterial load to quantify the effect on the pathogen, and advance to in vivo challenge. The protection shown here is cellular; efficacy in fish is the next question.

06 · Limitations

Scope of these data

Results correspond to one cell system (SHK-1) at the stated doses and time points; the demonstrated protection is in vitro and does not yet equal therapeutic efficacy in fish.

The supplied data do not include error bars, replicate counts, or statistical analysis, so comparisons are described as observed values. The increase for Candidate B at 1 µg/mL on day 10 requires confirmation. Cytotoxicity is an indirect infection readout and should be complemented by direct bacterial-load measurement.