Potentially very important study with large implications posted to medRxiv on April 22. @AaronRichterman medrxiv.org/content/10.110…
This group of researchers from Germany looked for T-cell responses to SARS-CoV-2 among 18 patients with COVID19 and 68 healthy controls. Characteristics of these individuals shown below 
They note that adaptive immune responses were found to be protective for SARS-CoV and suggest CD4 T cell responses may be important for generation of neutralizing antibodies
They extracted CD4+ T cells and analyzed their reactivity to S-protein. They note they found reactivity in 34% of healthy donors where reactivity was more likely to the C-terminal end of S2
They note the C-terminal end of S2 has the highest similarity to common human coronaviruses
They also note that among patients with COVID19 who did not have reactive T cells to S-protein, these tended to be sicker (critical patients). They also found evidence of T cells that are reactive to other SARSCoV2 proteins
Implications are many including a theoretical immunologic reason for some protection of younger individuals who might have cross-protection given more frequent exposure to other human coronaviruses. Also may be implications for some of the serologic tests that are coming on board
Finally, cross reactivity may be a reason to be hopeful about the prospect of a vaccine: we may not need perfect protection. Cross-immunity may provide enough protection, particularly if our vaccines can illicit a strong T cell response.
Additional point: there are some reports about differences in HLA types and risk for severe disease. The work described here might provide a further immunologic rationale for that. See: jvi.asm.org/content/early/…
