Tonix Pharmaceuticals Holding Corp. announced that the first participant was enrolled in a dose-finding study for TNX-2100 (SARS-CoV-2 epitope peptide mixtures for intradermal administration), an in vivo skin test to measure delayed-type hypersensitivity (DTH) to SARS-CoV-2 (CoV-2), the virus that causes COVID-19. DTH is a measure of functional T cell immunity.
Because antibodies are easier to measure, there has been more discussion about antibodies rather than T cells as a biomarker of immunity to SARS-CoV-2. However, T cells, not antibodies, are the body's major defense against viruses. The skin test has the potential to serve as: 1) a biomarker for T cell protective immunity and durability of vaccine protection; 2) a personalized approach for vaccine boosters; 3) a method to stratify participants in COVID-19 vaccine trials with a more complete picture of immune status; 4) an endpoint in COVID-19 vaccine trials for vaccines that elicit T cell immunity, and 5) public health surveillance. T cell immunity to SARS-CoV-2 is believed to provide an important element of protective immunity against serious COVID-19 illness after infection with SARS-CoV-2. T cell immunity to several viruses is known to persist substantially longer than antibody immunity, often present when there is no measurable antibody response, and in many cases provides a more durable protective immunity that may last for years to decades. For example, T cell
immunity has been detected to SARS-CoV-2 in some individuals who do not make antibody responses and in others after their antibody responses have waned and become undetectable over time. A complete picture of a
patient's immune status requires assessment of both antibody titers and T cell immunity. Skin tests that elicit DTH responses are well-established methods for the detection of antigen-specific T cell
responses. Tests of this kind are known as in vivo diagnostics because they require injection of small
representative peptides into the skin. A positive result is evidenced by a skin reaction surrounding the site
of the injection produced by local infiltration of functional antigen-specific T cells. This reaction, also
called “induration”, has been shown to be dependent on the presence of memory T cells. Both the CD4+ and
CD8+ fractions of T cells participate in this response.