[¹⁸F]FLT PET visualizes the immune response after vaccination. (A) Three days after intranodal delivery of the [¹¹¹In]/SPIO-labeled and antigen-loaded DC, [¹⁸F]FLT PET/CT scan was performed and showed tracer retention in the injected and three draining LNs. (B) Immediately before the PET/CT scan, the same LNs were visualized by scintigraphy, containing 4%, 3%, and 22% of the injected radioactivity. (C and D) During the same day, a radical LN dissection was performed and [¹¹¹In]-DC containing LNs were identified with a gamma probe. Immunohistochemical analyses of these LNs demonstrated a close interaction between injected SPIO-labeled DC and CD8⁺ T cells, resulting in increased expression of activation marker CD25. (E and F) To show that de novo immune responses can also be imaged with [¹⁸F]FLT, we performed [¹⁸F]FLT PET scans in one patient after the prime vaccination (pt 2). At day 5 after vaccination, [¹⁸F]FLT signal had increased from SUV_max 1.7–2.5, whereas the control lymph node that received vaccination with DC not loaded with antigen did not show an increase in [¹⁸F]FLT signal (SUV_max 0.9–0.8). (G and H) In patient 6, who received multiple vaccinations, a sustained [¹⁸F]FLT signal was detected up to 3 wk after the last vaccination, but not in the nonvaccinated control LN. (I–L) To find the optimal time point for [¹⁸F]FLT PET imaging, sequential [¹⁸F]FLT PET/CT scans were performed in four patients. We observed a profound increase in [¹⁸F]FLT signal in vaccinated LNs (filled squares) but not in control LNs (open squares) between days 3 and 6 after injection with antigen-loaded DC. However, patient 3 received vaccination with DC loaded with tumor-antigen but not the control-antigen KLH. All patients received no other vaccinations 6 mo before imaging.

Increased [¹⁸F]FLT PET signal directly correlates to antigen-specific immune responses. (A) A significant increase in [¹⁸F]FLT signal (P < 0.05) was detected in LNs after three intranodal vaccinations, except in one patient (pt 3). (B) In contrast, in control LNs, no increase in tracer retention was observed. (C) Control LNs consisted of LNs at the contralateral side, either nonvaccinated LN or LN vaccinated with saline or DCs not loaded with antigen. An increase of [¹⁸F]FLT signal was not observed in any of these cases. (D) Peripheral blood samples at the time point of the scan showed a significant correlation between the level of KLH-specific IgG antibodies and the intensity of [¹⁸F]FLT uptake in the LNs, P < 0.05. (E) Proliferation of peripheral blood mononuclear cells upon stimulation with KLH demonstrated a clear correlation between the intensity of [¹⁸F]FLT signal and the KLH-specific proliferation in vitro, P < 0.05. One patient (double square) showed high [¹⁸F]FLT uptake (SUV_max 7.8) without accompanying KLH specific proliferation (403 ± 76 cpm) but showed pronounced humoral response to KLH (KLH IgG 918 mg/L), involving also KLH-specific IgA and IgM. In a second patient (circle), the observed [¹⁸F]FLT PET signal (SUV_max 6.6) resulted from a modest KLH-specific IgG response (KLH IgG 118 mg/L) and a pronounced KLH-specific proliferation (45,030 ± 12,996 cpm).